test: close critical coverage gaps and harden PR review fixes

Critical bug fixes from PR review agents:
- client.py: eager asyncio.Lock init, Final[frozenset] for _SENSITIVE_KEYS,
  explicit 429 ToolError after retries exhausted, removed lazy _get_client_lock()
  and _RateLimiter._get_lock() patterns
- exceptions.py: use builtin TimeoutError (UP041), explicit handler before broad
  except so asyncio timeouts get descriptive messages
- docker.py: add update_all to DESTRUCTIVE_ACTIONS (was missing), remove dead
  _MUTATION_ACTIONS constant
- manager.py: _cap_log_content returns new dict (immutable), lock write to
  resource_data, clean dead task from active_subscriptions after loop exits
- diagnostics.py: fix inaccurate comment about semicolon injection guard
- health.py: narrow except ValueError in _safe_display_url, fix TODO comment

New test coverage (98 tests added, 529 → 598 passing):
- test_subscription_validation.py: 27 tests for _validate_subscription_query
  (security-critical allow-list, forbidden keyword guards, word-boundary test)
- test_subscription_manager.py: 12 tests for _cap_log_content
  (immutability, truncation, nesting, passthrough)
- test_client.py: +57 tests — _RateLimiter (token math, refill, sleep-on-empty),
  _QueryCache (TTL, invalidation, is_cacheable), 429 retry loop (1/2/3 failures)
- test_health.py: +10 tests for _safe_display_url (credential strip, port,
  path/query removal, malformed IPv6 → <unparseable>)
- test_notifications.py: +7 importance enum and field length validation tests
- test_rclone.py: +7 _validate_config_data security guard tests
- test_storage.py: +15 (tail_lines bounds, format_kb, safe_get)
- test_docker.py: update_all now requires confirm=True + new guard test
- test_destructive_guards.py: update audit to include update_all

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

tests/safety/test_destructive_guards.py

@@ -39,7 +39,7 @@ KNOWN_DESTRUCTIVE: dict[str, dict[str, set[str]]] = {
         "module": "unraid_mcp.tools.docker",
         "register_fn": "register_docker_tool",
         "tool_name": "unraid_docker",
-        "actions": {"remove"},
+        "actions": {"remove", "update_all"},
         "runtime_set": DOCKER_DESTRUCTIVE,
     },
     "vm": {
@@ -143,6 +143,7 @@ class TestDestructiveActionRegistries:
 _DESTRUCTIVE_TEST_CASES: list[tuple[str, str, dict]] = [
     # Docker
     ("docker", "remove", {"container_id": "abc123"}),
+    ("docker", "update_all", {}),
     # VM
     ("vm", "force_stop", {"vm_id": "test-vm-uuid"}),
     ("vm", "reset", {"vm_id": "test-vm-uuid"}),