feat(tests): add Blockbuster runtime gate for event-loop blocking IO (#3229)

* feat(tests): add Blockbuster runtime gate for event-loop blocking IO

Adds a strict runtime gate that fails CI when sync blocking IO calls run
on the asyncio event loop thread through DeerFlow business code.

Components:
- backend/tests/support/detectors/blocking_io_runtime.py — Blockbuster
  context scoped to `app.*` and `deerflow.*` so test infrastructure,
  pytest internals, and third-party libraries stay silent.
- backend/tests/blocking_io/conftest.py — pytest_runtest_protocol
  hookwrapper that wraps every item (setup + call + teardown) with the
  strict context. Respects `@pytest.mark.allow_blocking_io` opt-out.
- backend/tests/blocking_io/test_skills_load.py — regression anchor for
  the #1917 fix (asyncio.to_thread offload around
  LocalSkillStorage.load_skills).
- backend/tests/blocking_io/test_sqlite_lifespan.py — regression anchor
  for the #1912 fix (asyncio.to_thread offload around
  ensure_sqlite_parent_dir).
- backend/tests/blocking_io/test_gate_smoke.py — meta-test asserting the
  gate actually catches unoffloaded blocking IO and that the
  `@pytest.mark.allow_blocking_io` opt-out works.
- backend/Makefile — `make test-blocking-io` target.
- .github/workflows/backend-blocking-io-tests.yml — hard-fail PR gate on
  ubuntu-latest. Windows matrix deferred to follow-up.

Dependencies:
- blockbuster>=1.5.26,<1.6 added to dev group.

Coverage boundary (called out in PR body): the gate only catches blocking
IO on code paths the test suite actually exercises. Static AST inventory
(separate, informational) is the complementary coverage tool. Three blind
spot categories — untested paths, mocked-away paths, env-mismatched paths
— are documented in the PR description.

Findings surfaced while authoring this PR:
- resolve_sqlite_conn_str in runtime/store/_sqlite_utils.py:19 does sync
  Path.resolve() -> os.path.abspath on the lifespan loop thread, ahead of
  the #1912 fix. Not addressed here; tracked as follow-up.

Tests: 4 passed locally (`make test-blocking-io`).
Lint/format: clean (`ruff check` and `ruff format --check`).

* fix(tests): scope Blockbuster gate to blocking-io suite

* fix(tests): harden Blockbuster runtime gate

* test(blocking-io): add project rule extension point

* test(blocking-io): address review cleanup

backend/packages/harness/deerflow/runtime/checkpointer/async_provider.py

@@ -34,6 +34,19 @@ from deerflow.runtime.store._sqlite_utils import ensure_sqlite_parent_dir, resol
 
 logger = logging.getLogger(__name__)
 
+
+def _prepare_sqlite_checkpointer_path(raw: str) -> str:
+    conn_str = resolve_sqlite_conn_str(raw)
+    ensure_sqlite_parent_dir(conn_str)
+    return conn_str
+
+
+def _prepare_database_sqlite_checkpointer_path(db_config) -> str:
+    conn_str = db_config.checkpointer_sqlite_path
+    ensure_sqlite_parent_dir(conn_str)
+    return conn_str
+
+
 # ---------------------------------------------------------------------------
 # Async factory
 # ---------------------------------------------------------------------------
@@ -54,8 +67,7 @@ async def _async_checkpointer(config) -> AsyncIterator[Checkpointer]:
         except ImportError as exc:
             raise ImportError(SQLITE_INSTALL) from exc
 
-        conn_str = resolve_sqlite_conn_str(config.connection_string or "store.db")
-        await asyncio.to_thread(ensure_sqlite_parent_dir, conn_str)
+        conn_str = await asyncio.to_thread(_prepare_sqlite_checkpointer_path, config.connection_string or "store.db")
         async with AsyncSqliteSaver.from_conn_string(conn_str) as saver:
             await saver.setup()
             yield saver
@@ -98,8 +110,7 @@ async def _async_checkpointer_from_database(db_config) -> AsyncIterator[Checkpoi
         except ImportError as exc:
             raise ImportError(SQLITE_INSTALL) from exc
 
-        conn_str = db_config.checkpointer_sqlite_path
-        ensure_sqlite_parent_dir(conn_str)
+        conn_str = await asyncio.to_thread(_prepare_database_sqlite_checkpointer_path, db_config)
         async with AsyncSqliteSaver.from_conn_string(conn_str) as saver:
             await saver.setup()
             yield saver