[codex] Remove unused event messages (#20511)

## Why

Several legacy `EventMsg` variants were still emitted or mapped even
though clients either ignored them or had moved to item/lifecycle
events. `Op::Undo` had also degraded to an unavailable shim, so this
removes that dead task path instead of preserving a command that cannot
do useful work.

`McpStartupComplete`, `WebSearchBegin`, and `ImageGenerationBegin` are
intentionally kept because useful consumers still depend on them: MCP
startup completion drives readiness behavior, and the begin events let
app-server/core consumers surface in-progress web-search and
image-generation items before the final payload arrives.

## What Changed

- Removed weak legacy event variants and payloads from `codex-protocol`,
including legacy agent deltas, background events, and undo lifecycle
events.
- Kept/restored `EventMsg::McpStartupComplete`,
`EventMsg::WebSearchBegin`, and `EventMsg::ImageGenerationBegin` with
serializer and emission coverage.
- Updated core, rollout, MCP server, app-server thread history,
review/delegate filtering, and tests to rely on the useful replacement
events that remain.
- Removed `Op::Undo`, `UndoTask`, the undo test module, and stale TUI
slash-command comments.
- Stopped agent job/background progress and compaction retry notices
from emitting `BackgroundEvent` payloads.

## Verification

- `cargo check -p codex-protocol -p codex-app-server-protocol -p
codex-core -p codex-rollout -p codex-rollout-trace -p codex-mcp-server`
- `cargo test -p codex-protocol -p codex-app-server-protocol -p
codex-rollout -p codex-rollout-trace -p codex-mcp-server`
- `cargo test -p codex-core --test all suite::items`
- `just fix -p codex-protocol -p codex-app-server-protocol -p codex-core
-p codex-rollout -p codex-rollout-trace -p codex-mcp-server`
- Earlier coverage on this PR also included `codex-mcp`, `codex-tui`,
core library tests, MCP/plugin/delegate/review/agent job tests, and MCP
startup TUI tests.

codex-rs/protocol/src/protocol.rs

@@ -777,12 +777,6 @@ pub enum Op {
     /// model.
     SetThreadMemoryMode { mode: ThreadMemoryMode },
 
-    /// Legacy request to undo a turn.
-    ///
-    /// The op is still accepted for compatibility, but ghost snapshots are no
-    /// longer produced so the request reports unavailable.
-    Undo,
-
     /// Request Codex to drop the last N user turns from in-memory context.
     ///
     /// This does not attempt to revert local filesystem changes. Clients are
@@ -911,7 +905,6 @@ impl Op {
             Self::Compact => "compact",
             Self::SetThreadName { .. } => "set_thread_name",
             Self::SetThreadMemoryMode { .. } => "set_thread_memory_mode",
-            Self::Undo => "undo",
             Self::ThreadRollback { .. } => "thread_rollback",
             Self::Review { .. } => "review",
             Self::ApproveGuardianDeniedAction { .. } => "approve_guardian_denied_action",
@@ -1368,20 +1361,12 @@ pub enum EventMsg {
     /// User/system input message (what was sent to the model)
     UserMessage(UserMessageEvent),
 
-    /// Agent text output delta message
-    AgentMessageDelta(AgentMessageDeltaEvent),
-
     /// Reasoning event from agent.
     AgentReasoning(AgentReasoningEvent),
 
-    /// Agent reasoning delta event from agent.
-    AgentReasoningDelta(AgentReasoningDeltaEvent),
-
     /// Raw chain-of-thought from agent.
     AgentReasoningRawContent(AgentReasoningRawContentEvent),
 
-    /// Agent reasoning content delta event from agent.
-    AgentReasoningRawContentDelta(AgentReasoningRawContentDeltaEvent),
     /// Signaled when the model begins a new reasoning summary section (e.g., a new titled block).
     AgentReasoningSectionBreak(AgentReasoningSectionBreakEvent),
 
@@ -1447,12 +1432,6 @@ pub enum EventMsg {
     /// deprecated and should be phased out.
     DeprecationNotice(DeprecationNoticeEvent),
 
-    BackgroundEvent(BackgroundEventEvent),
-
-    UndoStarted(UndoStartedEvent),
-
-    UndoCompleted(UndoCompletedEvent),
-
     /// Notification that a model stream experienced an error or disconnect
     /// and the system is handling it (e.g., retrying with backoff).
     StreamError(StreamErrorEvent),
@@ -1894,9 +1873,7 @@ pub struct AgentMessageContentDeltaEvent {
 
 impl HasLegacyEvent for AgentMessageContentDeltaEvent {
     fn as_legacy_events(&self, _: bool) -> Vec<EventMsg> {
-        vec![EventMsg::AgentMessageDelta(AgentMessageDeltaEvent {
-            delta: self.delta.clone(),
-        })]
+        Vec::new()
     }
 }
 
@@ -1921,9 +1898,7 @@ pub struct ReasoningContentDeltaEvent {
 
 impl HasLegacyEvent for ReasoningContentDeltaEvent {
     fn as_legacy_events(&self, _: bool) -> Vec<EventMsg> {
-        vec![EventMsg::AgentReasoningDelta(AgentReasoningDeltaEvent {
-            delta: self.delta.clone(),
-        })]
+        Vec::new()
     }
 }
 
@@ -1940,11 +1915,7 @@ pub struct ReasoningRawContentDeltaEvent {
 
 impl HasLegacyEvent for ReasoningRawContentDeltaEvent {
     fn as_legacy_events(&self, _: bool) -> Vec<EventMsg> {
-        vec![EventMsg::AgentReasoningRawContentDelta(
-            AgentReasoningRawContentDeltaEvent {
-                delta: self.delta.clone(),
-            },
-        )]
+        Vec::new()
     }
 }
 
@@ -2309,11 +2280,6 @@ pub struct UserMessageEvent {
     pub text_elements: Vec<crate::user_input::TextElement>,
 }
 
-#[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS)]
-pub struct AgentMessageDeltaEvent {
-    pub delta: String,
-}
-
 #[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS)]
 pub struct AgentReasoningEvent {
     pub text: String,
@@ -2324,11 +2290,6 @@ pub struct AgentReasoningRawContentEvent {
     pub text: String,
 }
 
-#[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS)]
-pub struct AgentReasoningRawContentDeltaEvent {
-    pub delta: String,
-}
-
 #[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS)]
 pub struct AgentReasoningSectionBreakEvent {
     // load with default value so it's backward compatible with the old format.
@@ -2338,11 +2299,6 @@ pub struct AgentReasoningSectionBreakEvent {
     pub summary_index: i64,
 }
 
-#[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS)]
-pub struct AgentReasoningDeltaEvent {
-    pub delta: String,
-}
-
 #[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS, PartialEq)]
 pub struct McpInvocation {
     /// Name of the MCP server as defined in the config.
@@ -3188,11 +3144,6 @@ pub struct TerminalInteractionEvent {
     pub stdin: String,
 }
 
-#[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS)]
-pub struct BackgroundEventEvent {
-    pub message: String,
-}
-
 #[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS)]
 pub struct DeprecationNoticeEvent {
     /// Concise summary of what is deprecated.
@@ -3202,19 +3153,6 @@ pub struct DeprecationNoticeEvent {
     pub details: Option<String>,
 }
 
-#[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS)]
-pub struct UndoStartedEvent {
-    #[serde(skip_serializing_if = "Option::is_none")]
-    pub message: Option<String>,
-}
-
-#[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS)]
-pub struct UndoCompletedEvent {
-    pub success: bool,
-    #[serde(skip_serializing_if = "Option::is_none")]
-    pub message: Option<String>,
-}
-
 #[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS)]
 pub struct ThreadRolledBackEvent {
     /// Number of user turns that were removed from context.