fix(uv): update dependency pins and improve test reliability

[timothytlewis]

Summary

• Widen libgit2.org from ~1.7 to >=1.7<2 — allow current 1.8/1.9 ABI versions
• Widen cmake.org from ^3.28 to >=3.28 — allow CMake 4.x (current stable)
• Replace sleep 10 in test with retry loop for faster, more reliable test execution
• Add || true to kill $PID to prevent test failure if process already exited

Audit findings

• Important: libgit2.org: ~1.7 blocks 1.8.x and 1.9.x (current is 1.9.2)
• Important: cmake.org: ^3.28 blocks CMake 4.x (current is 4.2.3)
• Minor: sleep 10 is fragile; retry loop is more robust

Test plan

• Verify uv builds with libgit2 1.8+ or 1.9+
• Verify uv builds with CMake 4.x
• Verify test Flask server wait is reliable

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