Fix: Orphaned process and goroutine leak in ScriptRun/CustomSync leading to split-brain deployments

[pulkitvats2007-crypto]

What this PR does:

• Fixes goroutine and zombie process leaks in ScriptRun, CustomSync, and Kubernetes rollback executors.
• Binds external command execution to a cancellable context using exec.CommandContext.
• Ensures that timeout or cancellation properly terminates underlying OS processes.
• Adds cmd.WaitDelay = time.Second to prevent hangs caused by inherited pipes from background shell jobs.
• Updates all affected executors:
  • pkg/app/piped/executor/scriptrun/scriptrun.go
  • pkg/app/piped/executor/customsync/customsync.go
  • pkg/app/piped/executor/customsync/rollback.go
  • pkg/app/piped/executor/kubernetes/rollback.go
• Introduces tests (TestExecuteCommandCancellation) to verify proper cancellation behavior and prevent regressions.

Why we need it:

• Prevents concurrent deployment corruption caused by orphaned scripts continuing execution after cancellation.
• Eliminates goroutine leaks and zombie processes that can exhaust system resources (CPU, memory, file descriptors).
• Fixes silent failure scenarios where pipelines report "FAILED" or "CANCELLED" but scripts continue mutating infrastructure.
• Restores correctness and reliability of PipeCD’s cancellation and timeout mechanisms.

Which issue(s) this PR fixes:

Fixes #6734

Does this PR introduce a user-facing change?:

• No

• How are users affected by this change:

  • Users will experience correct and deterministic cancellation behavior.
  • Cancelled or timed-out stages will no longer continue executing in the background.
  • Improved system stability under frequent cancellations or retries.

• Is this breaking change:

  • No

• How to migrate (if breaking change):

  • N/A