test: remove dead stocktrim_client_with_mock_transport fixture

Addresses Copilot's review on #168: the fixture was already broken —
\`AuthenticatedClient._client\` is initialized to \`None\` and isn't
created until \`get_httpx_client()\` runs, so the original
\`client._client._transport = mock_transport\` assignment raised
\`AttributeError\`. My #168 \"fix\" replaced a mypy-style ignore with an
\`assert client._client is not None\`, but the assert would always fire
the moment anything actually used the fixture.

It never did: \`grep -rn stocktrim_client_with_mock_transport\` finds
zero callers anywhere in \`tests/\` or \`stocktrim_mcp_server/tests/\`.
That's why CI stayed green through both the original broken version
and my equally-broken \"fix\".

Deleting it rather than rewriting it via \`set_httpx_client\` /
\`set_async_httpx_client\`, since there's no consumer and we already
have working transport-mocking patterns (\`httpx.MockTransport\` via
\`mock_transport_handler\` / \`AsyncMockTransport\`) used by every actual
test that needs network isolation.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: dougborg

tests/conftest.py

@@ -214,13 +214,3 @@ def mock_not_found_response():
     response.headers = {"Content-Type": "application/problem+json"}
     response.text = '{"type": "...", "title": "Not Found", "status": 404}'
     return response
-
-
-@pytest.fixture
-def stocktrim_client_with_mock_transport(mock_api_credentials, mock_transport):
-    """Create a StockTrimClient with mock transport for testing without network calls."""
-    client = StockTrimClient(**mock_api_credentials)
-    # Replace the transport with mock
-    assert client._client is not None
-    client._client._transport = mock_transport
-    return client