core: bundle settings diff updates into one dev/user envelope (#12417)

## Summary
- bundle contextual prompt injection into at most one developer message
plus one contextual user message in both:
  - per-turn settings updates
  - initial context insertion
- preserve `<model_switch>` across compaction by rebuilding it through
canonical initial-context injection, instead of relying on
strip/reattach hacks
- centralize contextual user fragment detection in one shared definition
table and reuse it for parsing/compaction logic
- keep `AGENTS.md` in its natural serialized format:
  - `# AGENTS.md instructions for {dirname}`
  - `<INSTRUCTIONS>...</INSTRUCTIONS>`
- simplify related tests/helpers and accept the expected snapshot/layout
updates from bundled multi-part messages

## Why
The goal is to converge toward a simpler, more intentional prompt shape
where contextual updates are consistently represented as one developer
envelope plus one contextual user envelope, while keeping parsing and
compaction behavior aligned with that representation.

## Notable details
- the temporary `SettingsUpdateEnvelope` wrapper was removed; these
paths now return `Vec<ResponseItem>` directly
- local/remote compaction no longer rely on model-switch strip/restore
helpers
- contextual user detection is now driven by shared fragment definitions
instead of ad hoc matcher assembly
- AGENTS/user instructions are still the same logical context; only the
synthetic `<user_instructions>` wrapper was replaced by the natural
AGENTS text format

## Testing
- `just fmt`
- `cargo test -p codex-app-server
codex_message_processor::tests::extract_conversation_summary_prefers_plain_user_messages
-- --exact`
- `cargo test -p codex-core
compact::tests::collect_user_messages_filters_session_prefix_entries
--lib -- --exact`
- `cargo test -p codex-core --test all
'suite::compact::snapshot_request_shape_pre_turn_compaction_strips_incoming_model_switch'
-- --exact`
- `cargo test -p codex-core --test all
'suite::compact_remote::snapshot_request_shape_remote_pre_turn_compaction_strips_incoming_model_switch'
-- --exact`
- `cargo test -p codex-core --test all
'suite::client::includes_apps_guidance_as_developer_message_when_enabled'
-- --exact`
- `cargo test -p codex-core --test all
'suite::client::includes_developer_instructions_message_in_request' --
--exact`
- `cargo test -p codex-core --test all
'suite::client::includes_user_instructions_message_in_request' --
--exact`
- `cargo test -p codex-core --test all
'suite::client::resume_includes_initial_messages_and_sends_prior_items'
-- --exact`
- `cargo test -p codex-core --test all
'suite::review::review_input_isolated_from_parent_history' -- --exact`
- `cargo test -p codex-exec --test all
'suite::resume::exec_resume_last_respects_cwd_filter_and_all_flag' --
--exact`
- `cargo test -p core_test_support
context_snapshot::tests::full_text_mode_preserves_unredacted_text --
--exact`

## Notes
- I also ran several targeted `compact`, `compact_remote`,
`prompt_caching`, `model_visible_layout`, and `event_mapping` tests
while iterating on prompt-shape changes.
- I have not claimed a clean full-workspace `cargo test` from this
environment because local sandbox/resource conditions have previously
produced unrelated failures in large workspace runs.

codex-rs/core/tests/suite/compact.rs

@@ -756,16 +756,40 @@ async fn multiple_auto_compact_per_task_runs_after_token_limit_hit() {
     let body = requests_payloads[0].body_json();
     let input = body.get("input").and_then(|v| v.as_array()).unwrap();
 
+    fn strip_agents_parts_from_user_message(
+        value: &serde_json::Value,
+    ) -> Option<serde_json::Value> {
+        let content = value
+            .get("content")
+            .and_then(|content| content.as_array())?;
+        let filtered_content = content
+            .iter()
+            .filter(|item| {
+                !item
+                    .get("text")
+                    .and_then(|text| text.as_str())
+                    .is_some_and(|text| text.starts_with("# AGENTS.md instructions for "))
+            })
+            .cloned()
+            .collect::<Vec<_>>();
+        if filtered_content.is_empty() {
+            return None;
+        }
+        let mut normalized = value.clone();
+        normalized["content"] = serde_json::Value::Array(filtered_content);
+        Some(normalized)
+    }
+
     fn normalize_inputs(values: &[serde_json::Value]) -> Vec<serde_json::Value> {
         values
             .iter()
-            .filter(|value| {
+            .filter_map(|value| {
                 if value
                     .get("type")
                     .and_then(|ty| ty.as_str())
                     .is_some_and(|ty| ty == "function_call_output")
                 {
-                    return false;
+                    return None;
                 }
 
                 let text = value
@@ -781,11 +805,13 @@ async fn multiple_auto_compact_per_task_runs_after_token_limit_hit() {
                 if role == Some("developer")
                     && text.is_some_and(|text| text.contains("`sandbox_mode`"))
                 {
-                    return false;
+                    return None;
                 }
-                !text.is_some_and(|text| text.starts_with("# AGENTS.md instructions for "))
+                if role == Some("user") {
+                    return strip_agents_parts_from_user_message(value);
+                }
+                Some(value.clone())
             })
-            .cloned()
             .collect()
     }
 
@@ -3184,7 +3210,7 @@ async fn snapshot_request_shape_pre_turn_compaction_context_window_exceeded() {
     ]);
     let mut responses = vec![first_turn];
     responses.extend(
-        (0..6).map(|_| {
+        (0..5).map(|_| {
             sse_failed(
                 "compact-failed",
                 "context_length_exceeded",