Add text element metadata to protocol, app server, and core (#9331)

The second part of breaking up PR
https://github.com/openai/codex/pull/9116

Summary:

- Add `TextElement` / `ByteRange` to protocol user inputs and user
message events with defaults.
- Thread `text_elements` through app-server v1/v2 request handling and
history rebuild.
- Preserve UI metadata only in user input/events (not `ContentItem`)
while keeping local image attachments in user events for rehydration.

Details:

- Protocol: `UserInput::Text` carries `text_elements`;
`UserMessageEvent` carries `text_elements` + `local_images`.
Serialization includes empty vectors for backward compatibility.
- app-server-protocol: v1 defines `V1TextElement` / `V1ByteRange` in
camelCase with conversions; v2 uses its own camelCase wrapper.
- app-server: v1/v2 input mapping includes `text_elements`; thread
history rebuilds include them.
- Core: user event emission preserves UI metadata while model history
stays clean; history replay round-trips the metadata.

codex-rs/app-server/tests/suite/codex_message_processor_flow.rs

@@ -114,6 +114,7 @@ async fn test_codex_jsonrpc_conversation_flow() -> Result<()> {
             conversation_id,
             items: vec![codex_app_server_protocol::InputItem::Text {
                 text: "text".to_string(),
+                text_elements: Vec::new(),
             }],
         })
         .await?;
@@ -241,6 +242,7 @@ async fn test_send_user_turn_changes_approval_policy_behavior() -> Result<()> {
             conversation_id,
             items: vec![codex_app_server_protocol::InputItem::Text {
                 text: "run python".to_string(),
+                text_elements: Vec::new(),
             }],
         })
         .await?;
@@ -296,6 +298,7 @@ async fn test_send_user_turn_changes_approval_policy_behavior() -> Result<()> {
             conversation_id,
             items: vec![codex_app_server_protocol::InputItem::Text {
                 text: "run python again".to_string(),
+                text_elements: Vec::new(),
             }],
             cwd: working_directory.clone(),
             approval_policy: AskForApproval::Never,
@@ -405,6 +408,7 @@ async fn test_send_user_turn_updates_sandbox_and_cwd_between_turns() -> Result<(
             conversation_id,
             items: vec![InputItem::Text {
                 text: "first turn".to_string(),
+                text_elements: Vec::new(),
             }],
             cwd: first_cwd.clone(),
             approval_policy: AskForApproval::Never,
@@ -437,6 +441,7 @@ async fn test_send_user_turn_updates_sandbox_and_cwd_between_turns() -> Result<(
             conversation_id,
             items: vec![InputItem::Text {
                 text: "second turn".to_string(),
+                text_elements: Vec::new(),
             }],
             cwd: second_cwd.clone(),
             approval_policy: AskForApproval::Never,