[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/unified_exec/mod.rs

@@ -68,9 +68,6 @@ pub(crate) const UNIFIED_EXEC_OUTPUT_MAX_BYTES: usize = 1024 * 1024; // 1 MiB
 pub(crate) const UNIFIED_EXEC_OUTPUT_MAX_TOKENS: usize = UNIFIED_EXEC_OUTPUT_MAX_BYTES / 4;
 pub(crate) const MAX_UNIFIED_EXEC_PROCESSES: usize = 64;
 
-// Send a warning message to the models when it reaches this number of processes.
-pub(crate) const WARNING_UNIFIED_EXEC_PROCESSES: usize = 60;
-
 pub(crate) struct UnifiedExecContext {
     pub session: Arc<Session>,
     pub turn: Arc<TurnContext>,

codex-rs/core/src/unified_exec/process_manager.rs

@@ -37,7 +37,6 @@ use crate::unified_exec::ProcessStore;
 use crate::unified_exec::UnifiedExecContext;
 use crate::unified_exec::UnifiedExecError;
 use crate::unified_exec::UnifiedExecProcessManager;
-use crate::unified_exec::WARNING_UNIFIED_EXEC_PROCESSES;
 use crate::unified_exec::WriteStdinRequest;
 use crate::unified_exec::async_watcher::emit_exec_end_for_unified_exec;
 use crate::unified_exec::async_watcher::emit_failed_exec_end_for_unified_exec;
@@ -827,11 +826,11 @@ impl UnifiedExecProcessManager {
             session: Arc::downgrade(&context.session),
             last_used: started_at,
         };
-        let (number_processes, pruned_entry) = {
+        let pruned_entry = {
             let mut store = self.process_store.lock().await;
             let pruned_entry = Self::prune_processes_if_needed(&mut store);
             store.processes.insert(process_id, entry);
-            (store.processes.len(), pruned_entry)
+            pruned_entry
         };
         // prune_processes_if_needed runs while holding process_store; do async
         // network-approval cleanup only after dropping that lock.
@@ -840,16 +839,6 @@ impl UnifiedExecProcessManager {
             pruned_entry.process.terminate();
         }
 
-        if number_processes >= WARNING_UNIFIED_EXEC_PROCESSES {
-            context
-                .session
-                .record_model_warning(
-                    format!("The maximum number of unified exec processes you can keep open is {WARNING_UNIFIED_EXEC_PROCESSES} and you currently have {number_processes} processes open. Reuse older processes or close them to prevent automatic pruning of old processes"),
-                    &context.turn
-                )
-                .await;
-        };
-
         spawn_exit_watcher(
             Arc::clone(&process),
             Arc::clone(&context.session),