Fix external repo resolver process cleanup

[sharmila-oai]

Why

When ExternalRepoResolver runs bazel query to resolve canonical bzlmod external repo paths, it reads only the first line and returns immediately. Without explicitly closing stdin/stderr and terminating the child process, the spawned Bazel query can continue running in the background. This can leak subprocesses and create avoidable resource pressure during fine-grained external repo hashing.

What changed

• Updated runProcessAndCaptureFirstLine to:
  • discard stderr output
  • close process stdin immediately
  • read one stdout line and then always clean up the process
• Added terminateProcess helper that:
  • attempts graceful termination (destroy)
  • escalates to force kill (destroyForcibly) if needed

Validation

• bazel test //cli:SourceFileHasherTest
• Previously validated with bazel test //cli:SourceFileHasherTest //cli:E2ETest for this exact patch