feat: static curl build + POST streaming transport test

[0xeb]

Summary

• Add CMake option to fetch and statically link libcurl (avoids DLL dependency issues on Windows)
• Fix POST streaming transport error handling
• Add POST streaming transport integration test

Changes

• cmake: fetch libcurl when requested
• cmake: make fetched curl static + fix POST stream error
• build(windows): avoid libcurl-d.dll loader errors
• build: always avoid curl DLL dependency
• cmake: always fetch static curl when requested
• test(post-stream): add POST streaming transport test

Test plan

• CI passes on all platforms
• POST streaming test passes