[codex] Filter legacy warning messages during compaction (#22243)

## Why

Older sessions can contain model-warning records persisted as `user`
messages, including the unified exec process-limit warning, the
`apply_patch`-via-`exec_command` warning, and the model-mismatch
high-risk cyber fallback warning. Those warnings are no longer produced
as conversation history items, but when old sessions compact they should
still be recognized as injected context rather than preserved as real
user turns.

## What changed

- Removed `record_model_warning` and the production paths that emitted
these warning messages into conversation history.
- Added `LegacyUnifiedExecProcessLimitWarning`,
`LegacyApplyPatchExecCommandWarning`, and `LegacyModelMismatchWarning`
contextual fragments that are used only for matching old persisted
messages.
- Registered the legacy fragments with contextual user message detection
so compaction filters them through the existing fragment path.
- Added focused compaction coverage for old warning messages being
dropped during compacted-history processing.

## Testing

- `cargo test -p codex-core warning`
- `just fix -p codex-core`

codex-rs/core/src/session/mod.rs

@@ -2436,22 +2436,6 @@ impl Session {
         state.record_items(items.iter(), turn_context.truncation_policy);
     }
 
-    pub(crate) async fn record_model_warning(&self, message: impl Into<String>, ctx: &TurnContext) {
-        self.services
-            .session_telemetry
-            .counter("codex.model_warning", /*inc*/ 1, &[]);
-        let item = ResponseItem::Message {
-            id: None,
-            role: "user".to_string(),
-            content: vec![ContentItem::InputText {
-                text: format!("Warning: {}", message.into()),
-            }],
-            phase: None,
-        };
-
-        self.record_conversation_items(ctx, &[item]).await;
-    }
-
     async fn maybe_warn_on_server_model_mismatch(
         self: &Arc<Self>,
         turn_context: &Arc<TurnContext>,
@@ -2488,8 +2472,6 @@ impl Session {
             }),
         )
         .await;
-        self.record_model_warning(warning_message, turn_context)
-            .await;
         true
     }