chore(photon): clean up ty type-checker warnings from lint-diff bot

The advisory lint-diff bot flagged 17 new ty diagnostics. 6 are
`unresolved-import` for httpx/aiohttp/pytest, which is structural
(CI lint env has no project deps) and matches every other platform
plugin's noise floor. The remaining 11 are real and fixable:

- `Optional[callable]` → `Optional[Callable[..., None]]` (auth.py)
  invalid-type-form on `callable` as a type expression. Added the
  proper `typing.Callable` import. Two sites: on_pending in
  poll_for_token, on_user_code in login_device_flow.

- Dropped three unused `# type: ignore` comments on
  hermes_constants / hermes_cli.config imports — ty can resolve
  those modules fine, the comments were dead.

- _supervise_sidecar(proc) widened `proc.stdout` from
  `IO[Any] | None` to a narrowed local after an early `is None`
  guard. Defensive against subprocesses launched without
  stdout=PIPE.

- cli.py _cmd_setup: dropped the `has_existing_project = bool(...)`
  intermediate, did the narrowing inline with `if existing_id and
  existing_secret:` so ty can see project_id/project_secret are
  non-None when create_user is called.

- test_inbound.py: replaced three `adapter.handle_message =
  fake_handle  # type: ignore[assignment]` with
  `monkeypatch.setattr(adapter, 'handle_message', fake_handle)`.
  Same behavior, no type-ignore, and the monkeypatch reverts
  cleanly between tests.

Validation:
  ty check plugins/platforms/photon/ tests/plugins/platforms/photon/
    → All checks passed!
  tests/plugins/platforms/photon/ → 26/26 pass
  py_compile clean
  Windows footgun checker → 0 footguns

plugins/platforms/photon/adapter.py

@@ -513,10 +513,13 @@ class PhotonAdapter(BasePlatformAdapter):
 
     async def _supervise_sidecar(self, proc: subprocess.Popen) -> None:
         """Pump the sidecar's stdout/stderr into our logger."""
+        if proc.stdout is None:  # subprocess was launched without stdout=PIPE
+            return
+        stdout = proc.stdout
         loop = asyncio.get_event_loop()
         try:
             while True:
-                line = await loop.run_in_executor(None, proc.stdout.readline)
+                line = await loop.run_in_executor(None, stdout.readline)
                 if not line:
                     break
                 logger.info("[photon-sidecar] %s", line.decode("utf-8", "replace").rstrip())