feat(in_exec): add command_timeout option to limit child process exec…

[zoklk]

Which issue(s) this PR fixes:
Fixes #5319

What this PR does / why we need it:
Adds command_timeout option to in_exec plugin, mirroring the existing
command_timeout option in out_exec.

Currently, in_exec calls child_process_execute without wait_timeout,
defaulting to nil (infinite wait). When the external command hangs,
Fluentd repeatedly shows previous child process is still running. skipped.
with no indication of the actual cause, making troubleshooting difficult.

out_exec already exposes command_timeout which maps to wait_timeout
in child_process_execute. This PR resolves the inconsistency between
the two plugins.

Docs Changes:
fluent/fluentd-docs-gitbook#619

Release Note:
Add command_timeout option to in_exec plugin to kill long-running
child processes and align behavior with out_exec.

[Watson1978]

Hi @zoklk, thanks for this great PR!
Could you also send a PR to the fluentd-docs-gitbook repository to add the documentation for the new command_timeout parameter instead of following?

**Docs Changes**:
The `command_timeout` option should be added to the in_exec documentation
at https://docs.fluentd.org/input/exec

[zoklk]

Hi @Watson1978 , I've sent a PR to the fluentd-docs-gitbook repository for the command_timeout documentation.

fluent/fluentd-docs-gitbook#619

[Watson1978]

@zoklk Thank you so much!

[Watson1978]

Looks good to me.

@kenhys Can you also review this PR?

[kenhys]

waiting CI...

[kenhys]

@zoklk Thanks!