HTTP/SSE auth hardening and test file split

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Summary

• Split large interactions.cpp (5500+ lines) into 3 parts to fix MSVC heap exhaustion on Windows CI
• Added shared test fixture header for cleaner code organization
• Added HTTP/SSE server authentication hardening
• Added auth-related tests

Changes

• tests/server/interactions.cpp → split into interactions_p1.cpp, interactions_p2.cpp, interactions_p3.cpp
• New tests/server/interactions_fixture.hpp for shared test infrastructure
• New tests/server/http_auth.cpp for auth tests
• HTTP/SSE server hardening in http_server.cpp and sse_server.cpp

Test plan

• Windows CI passes (previously failing with heap exhaustion)
• Linux CI passes
• macOS CI passes
• All 164 interaction tests still pass across the 3 split files

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Pulling offline to fix Windows heap issues - need finer split