fix(client): validate bearer auth for async web search and fetch

[makiaveli1]

Client.web_search and Client.web_fetch raise a ValueError when no Bearer token is configured:

if not self._client.headers.get('authorization', '').startswith('Bearer '):
  raise ValueError('Authorization header with Bearer token is required for web search')

The AsyncClient versions of both methods omitted that check (and the matching Raises: docstring line), so an async caller without OLLAMA_API_KEY does not get the documented error. The request goes out and comes back as a ResponseError instead:

ollama._types.ResponseError: Unauthorized (status code: 401)

The async methods now perform the same check as their sync counterparts.

Verification:

• python -m pytest tests/ -q -> 99 passed
• Two new tests, test_async_client_web_search_requires_bearer_auth_header and test_async_client_web_fetch_requires_bearer_auth_header, mirror the existing sync tests. Both fail against the current code with the 401 above, and pass with this change.
• ruff check and ruff format --check -> clean

AI assistance: the analysis, fix and tests were prepared with an AI coding agent at my direction. I reproduced the behaviour locally, reviewed the change, and ran the checks above.