ci: enhance extensions test workflow

[standard-librarian]

Summary

This PR addresses #1042 by improving .github/workflows/extensions-test.yml:

• adds Cargo dependency caching (Swatinem/rust-cache@v2)
• introduces a shared build-libsql job and fans out test jobs via artifacts
• adds macOS and Windows runner coverage via a cross-platform build matrix

Details

• c-tests, rs-tests, and extensions-tests now depend on shared build output instead of each rebuilding libsql
• shared build artifacts are uploaded once and downloaded by dependent jobs
• kept existing test logic intact while reducing redundant build work

Closes #1042

[standard-librarian]

Pushed another fix for the persistent Windows ./configure failure.

What changed:

• Ensured the LF git config step is in the cross-platform-build-checks matrix job (the Windows path).
• Added explicit line-ending normalization before configure on Windows:
  • sed -i 's/\r$//' configure VERSION

Latest commit: 16ace09