## Why
Concurrent TLS client builds can independently load the same platform trust store. Share overlapping loads while preserving fresh trust reads for later builds.
## What changed
- Share one in-flight native root load without reusing completed loads.
- Bypass sharing when `SSL_CERT_FILE` or `SSL_CERT_DIR` is set, and continue reading custom CA bundles independently for every build.
## Testing
Add tests for overlapping loads, fresh trust after completion, and recovery from a panicking loader. Extend TLS handshake coverage to verify that replacing a custom CA file affects new configurations while existing configurations retain their trust, and that empty or removed files produce errors.
GitOrigin-RevId: 1465ef3faf82f53651d9dbfe314341b17346580b