ensure thread.history_mode is immutable (#30261)

## Description

This PR makes `thread.history_mode` immutable after the thread's
canonical first `SessionMeta` has been written. Later same-thread
`SessionMeta` lines are compatibility metadata writes, not a new thread
definition.

Without this, an older binary could append a `SessionMeta` that omits
`history_mode`; when a newer binary replays it, serde defaults that
missing field to `legacy` and SQLite could downgrade a paginated thread.

## Why

`history_mode` is the persisted thread storage contract.
Paginated-thread fail-closed behavior and SQLite memory filtering depend
on it staying aligned with canonical rollout metadata, especially when
multiple Codex binary versions can touch the same local rollout.

## What changed

- Stop generic rollout metadata replay from overwriting `history_mode`
from later `SessionMeta` items.
- Remove `history_mode` from `ThreadMetadataPatch`, so mutable metadata
sync and app-server metadata updates cannot rewrite it.
- When local metadata sync has to recreate a missing SQLite row, recover
`history_mode` from the rollout's canonical first `SessionMeta` instead
of from a mutable patch.
- Keep the in-memory thread store using the created thread's canonical
`history_mode` instead of metadata patches.
- Fill the one remaining core test `CreateThreadParams` initializer with
the new `history_mode` field; Bazel CI caught this after the parent
history-mode PR landed.

## Validation

- `just fmt`
- `just test -p codex-thread-store`
- `just test -p codex-state
session_meta_does_not_set_model_or_reasoning_effort`

codex-rs/thread-store/src/thread_metadata_sync.rs

@@ -76,7 +76,6 @@ impl ThreadMetadataSync {
             cli_version: Some(env!("CARGO_PKG_VERSION").to_string()),
             git_info: git_info.map(git_info_patch_from_observation),
             memory_mode: Some(params.metadata.memory_mode),
-            history_mode: Some(params.history_mode),
             ..Default::default()
         };
         Self {
@@ -238,7 +237,6 @@ impl ThreadMetadataSync {
                     {
                         update.memory_mode = Some(memory_mode);
                     }
-                    update.history_mode = Some(meta_line.meta.history_mode);
                 }
                 RolloutItem::TurnContext(turn_ctx) => {
                     if !self.cwd_seen {
@@ -377,7 +375,6 @@ fn update_has_metadata_facts(update: &ThreadMetadataPatch) -> bool {
         || update.first_user_message.is_some()
         || update.git_info.is_some()
         || update.memory_mode.is_some()
-        || update.history_mode.is_some()
 }
 
 fn git_info_patch_from_observation(git_info: GitInfo) -> GitInfoPatch {