feat(cli): add keep_latest prune strategy to ThreadTTLConfig (#6784)

## Summary
- Add `"keep_latest"` to `ThreadTTLConfig.strategy` to match
langgraph-api support for pruning old checkpoints while retaining the
thread and its latest state
- Add `sweep_limit` to `ThreadTTLConfig` where the API actually reads it
(was previously a no-op on `CheckpointerConfig`)
- Regenerate `schema.json` / `schema.v0.json`

## Test plan
- [x] `make format && make lint` passes
- [x] `make test` passes (85/85)

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

libs/cli/langgraph_cli/config.py

@@ -196,6 +196,11 @@ def validate_config(config: Config) -> Config:
                 f"Python version {pyversion} is not supported. "
                 f"Minimum required version is {MIN_PYTHON_VERSION}."
             )
+        if "bullseye" in pyversion:
+            raise click.UsageError(
+                "Bullseye images were deprecated in version 0.4.13. "
+                "Please use 'bookworm' or 'debian' instead."
+            )
 
         if not config["dependencies"]:
             raise click.UsageError(