Refactor stdout and stderr handling in BuckdLifecycle

[dkranchii]

Summary

Simplified error handling for child process stdout and stderr by replacing unwrap() with the ? operator for better error propagation.

• Replace .ok_or_else(|| internal_error!(...)).unwrap() with
  .ok_or_else(|| internal_error!(...))? in start_server_unix
• The function already returns buck2_error::Result<()>, so ?
  propagates the error gracefully instead of panicking the client
• The same function already uses ? on the line immediately above
  (cmd.spawn()?) — this change makes the error handling consistent

Test plan

• cargo check -p buck2_client_ctx — compiles cleanly
• cargo test -p buck2_client_ctx — all tests pass
• Behavioral: if stdout/stderr pipes are unexpectedly missing after
  spawn, the client now returns a descriptive error instead of panicking

[meta-codesync]

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