feat(state.db): persist platform_message_id; restore yuanbao exact-id recall

PR #29211 dropped JSONL gateway transcripts and noted that the platform's
own `message_id` field (used by Yuanbao's recall guard to redact a
message by exact platform id) was no longer preserved — falling back to
content-match.  That fallback works for the common case but redacts the
wrong row when two messages share text (or fails to match when content
is post-processed).

Restore exact-id matching by giving state.db a column for it:

- New `platform_message_id TEXT` column on the messages table
  (SCHEMA_VERSION bump 11 → 12; column added via declarative reconciler
  on existing DBs, no version-gated migration block needed)
- Partial index `idx_messages_platform_msg_id` on
  (session_id, platform_message_id) to keep recall's point-lookup cheap
  even on large sessions
- `append_message()` and `replace_messages()` accept the new value:
  the gateway-facing `append_to_transcript` in `gateway/session.py`
  forwards either `message["platform_message_id"]` or the legacy
  `message["message_id"]` key (yuanbao's existing convention)
- `get_messages_as_conversation()` surfaces the column back on the
  message dict as `message_id` so platform code reads the same shape
  it used to read from JSONL
- Yuanbao `_patch_transcript`: restore branch A1 (exact id match)
  ahead of A2 (content match) ahead of B (system-note).  Both branches
  log which one fired so operators can tell from gateway.log whether
  recall hit the canonical path or had to fall back.

Tests:
- New low-level round-trip tests in `test_hermes_state.py` for both
  `append_message` and `replace_messages` paths
- The PR's `test_yuanbao_recall_db_only.py` was rewritten to assert
  the new contract: branch A1 (id match) works against DB-only
  transcripts, and branch A2 (content match) still recovers rows that
  were observed without a platform id (e.g. agent-processed @bot
  messages where run.py doesn't carry msg_id through)

gateway/session.py

@@ -1271,6 +1271,12 @@ class SessionStore:
                     reasoning_details=message.get("reasoning_details") if message.get("role") == "assistant" else None,
                     codex_reasoning_items=message.get("codex_reasoning_items") if message.get("role") == "assistant" else None,
                     codex_message_items=message.get("codex_message_items") if message.get("role") == "assistant" else None,
+                    # Platform-side message id (yuanbao msg_id, telegram update_id, …).
+                    # Accept either explicit ``platform_message_id`` or the legacy
+                    # ``message_id`` key the JSONL transcript used.
+                    platform_message_id=(
+                        message.get("platform_message_id") or message.get("message_id")
+                    ),
                 )
             except Exception as e:
                 logger.debug("Session DB operation failed: %s", e)