Fix MCP auth bypass by failing on missing credentials

[DavidJBianco]

Motivation

• Authentication failures were returning empty headers which let _connect_http_server/SSE treat missing credentials as a successful connection and record unauthenticated clients.
• The goal is to restore a clear failure signal when configured authentication is missing or incomplete so connections requiring auth are not created without credentials.

Description

• Changed MCPClientManager._get_auth_headers return type to Optional[Dict[str, str]] and made it return None on auth failures (missing bearer token, incomplete API key config, or unavailable OAuth credentials) while still returning {} for servers with no auth configured.
• Updated HTTP connection gating in _connect_http_server to abort when _get_auth_headers returns None.
• Updated OAuth-related unit tests in tests/unit_tests/test_mcp_oauth_env_vars.py to expect None for auth-failure cases instead of an empty dict.
• Modified files: peak_assistant/utils/mcp_config.py and tests/unit_tests/test_mcp_oauth_env_vars.py.

Testing

• Ran python -m py_compile peak_assistant/utils/mcp_config.py tests/unit_tests/test_mcp_oauth_env_vars.py which succeeded.
• Ran pytest -q tests/unit_tests/test_mcp_oauth_env_vars.py and PYTHONPATH=. pytest -q tests/unit_tests/test_mcp_oauth_env_vars.py, both of which could not complete in this environment due to a missing dependency (python-dotenv).
• The changed unit assertions were updated to match the new None failure contract; remaining test failures are environmental (missing package) rather than regressions in the change set.

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Codex Task

[DavidJBianco]

Superseded by #85, which cherry-picked this fix. Merged into dev via merge commit 27a476d.