perf: cut startup latency — lazy import surface, langgraph dev keepalive, indexed thread listing (#407)

* feat: add thread metadata index for improved performance in thread listing

- Implemented a new SQLite index on the `checkpoints` table to optimize thread listing queries by indexing relevant metadata fields.
- Updated the `list_threads` function to ensure the index is created if it does not exist.
- Added a test to verify the creation of the metadata index during thread listing.

feat: enhance workspace sidecar management with owner tracking

- Modified the workspace sidecar to include `owner_pids` to track the current process owners.
- Updated tests to validate the new owner tracking functionality and ensure proper behavior when managing workspace sidecars.

chore: introduce model registry for streamlined model management

- Created a new `registry.py` file to maintain a comprehensive model registry, including model names, IDs, providers, and routing tables.
- Added functions to retrieve models by provider and list available models, enhancing the modularity and maintainability of model management.

* feat: enhance workspace sidecar management and improve thread metadata indexing

* fix(tests): ensure sidecar correctly registers owner with original workspace and pid

* refactor: simplify workspace sidecar management by removing owner tracking

* feat(server): add commands to manage background langgraph dev server

- Introduced `server_app` for managing the langgraph dev server with commands to check status and stop the server.
- Enhanced workspace sidecar management to include configuration fingerprint for drift detection.
- Updated deployment functions to handle server configuration and state more effectively.

* feat(server): enhance server status command to display PID with stale record warning

* feat(langgraph_dev): exclusion-set config fingerprint, webui keepalive, unified stop guidance

* fix(cli): platform-specific manual-stop hint; document keepalive endpoint-change limitation

tests/test_cli_deploy.py

@@ -134,6 +134,7 @@ def _run_deploy_once(
         jobs_per_worker=10,
         deploy_mode=False,
         tunnel=False,
+        config_fingerprint=None,
     ):
         captured["langgraph_dev_started"] = True
         captured["workspace_passed"] = str(workspace_dir) if workspace_dir else None