fix: address final review findings for service clients plan

Fixes 9 numbered findings + 7 minor fixes from the whole-plan review of
the MyWhoosh/Garmin service clients (Plan 3):

Garmin uploader (app/garmin/uploader.py):
- Detect Garmin-rejected imports (failures without successes) and raise
  new GarminImportRejected instead of reporting them as successful.
- Reclassify 429/rate-limit/500 login failures as transient instead of
  falling through to permanent auth errors; unrecognized login failures
  are now treated as transient (retryable) rather than GarminAuthError.
- Mirror the auth-token check from the login branch into the import
  branch so 401-at-import-time raises GarminAuthError instead of
  propagating raw.
- Add common GarminError base class, hoist transient-token tuple to a
  shared module constant, check response.status_code==409 before the
  duplicate substring fallback, and create the tokenstore dir 0o700.

MyWhoosh client (app/mywhoosh/client.py):
- Add optional max_pages bound to list_activities pagination.
- Add aclose()/__aenter__/__aexit__ so the client's own httpx.AsyncClient
  gets closed, while never closing an injected client.
- Guard the two remaining unguarded JSON-decode paths (login body,
  download-fit metadata) so malformed bodies raise
  MyWhooshIntegrationError instead of raw ValueError/AttributeError.
- Row-level malformation (missing id/activityFileId, unparseable
  startDatetime) is now skipped rather than aborting the whole page;
  envelope-shape failures still raise. id/activityFileId checks use
  explicit None/"" comparisons instead of Python falsiness.
- Replace asserts in _authenticated_post with explicit exceptions;
  restrict the reauth retry to 401 only, treat 403 as immediately
  terminal; naive startDatetime values are now treated as already-UTC
  instead of host-local.

MyWhoosh tokenstore (app/mywhoosh/tokenstore.py):
- load() now treats any corrupt/malformed token file (bad JSON, missing
  keys, OS errors) as "absent" instead of raising, so a bad cache no
  longer permanently wedges a user.

pyproject.toml:
- Tighten garminconnect pin to >=0.3.10,<1 (import_activity requires
  0.3.10+).

Adds/updates tests across tests/mywhoosh/ and tests/garmin/ covering
all of the above, including a fake client that wraps GarminUploadBlocked
in a plain RuntimeError to mirror the real garminconnect library's MFA
error wrapping.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/mywhoosh/test_tokenstore.py

@@ -16,3 +16,28 @@ def test_tokenstore_round_trip_and_permissions(tmp_path: Path) -> None:
 def test_missing_token_returns_none(tmp_path: Path) -> None:
     store = MyWhooshTokenStore(tmp_path / "missing.json")
     assert store.load() is None
+
+
+def test_corrupt_token_file_returns_none(tmp_path: Path) -> None:
+    path = tmp_path / "mywhoosh.json"
+    path.write_bytes(b"not valid json {{{")
+    store = MyWhooshTokenStore(path)
+    assert store.load() is None
+
+
+def test_token_file_missing_access_token_returns_none(tmp_path: Path) -> None:
+    path = tmp_path / "mywhoosh.json"
+    path.write_text('{"refresh_token": "r", "whoosh_id": "w"}', encoding="utf-8")
+    store = MyWhooshTokenStore(path)
+    assert store.load() is None
+
+
+def test_clear_removes_token_and_load_returns_none(tmp_path: Path) -> None:
+    store = MyWhooshTokenStore(tmp_path / "tokens" / "mywhoosh.json")
+    token = MyWhooshToken(access_token="access", refresh_token="refresh", whoosh_id="whoosh-1")
+    store.save(token)
+
+    store.clear()
+
+    assert store.load() is None
+    assert not store.path.exists()