Fix hanging MCP auth integration tests

[speterkins]

Summary

• switch the hanging MCP auth integration tests off the SSE transport
• align stale auth expectations with the current open-auth behavior
• keep the fix isolated from PR Relax default MCP auth enforcement #45 so it can land independently

Testing

• uv run pytest tests/test_north_mcp_server.py -q
• uv run pytest -q

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Note

Low Risk
Test-only changes that adjust transport and request shape; low risk aside from possibly masking transport-specific auth issues on SSE.

Overview
Auth integration tests are switched from the sse transport/GET /sse checks to streamable-http using POST /mcp JSON-RPC initialize requests to avoid hangs.

The test fixtures are simplified (removing the extra MCP client fixture), and a stale test expectation is renamed/updated to assert 401 for an empty Authorization header.

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