[Enhance](deploy) Support foreground mode for FDB in Docker environment

[Kekdeng]

In containerized deployments (like Docker env), fdbmonitor must run in the foreground so that the process remains attached to PID 1 and receives system signals correctly. The original fdb_ctl.sh only supported daemonized startup, which caused FDB to exit immediately when the container started. In addition, the existing initialization logic had inconsistent behaviors between foreground and background modes, and did not correctly propagate process exit codes to the container runtime.

This patch introduces minimal modifications to enable fully functional foreground mode while keeping compatibility with existing daemon mode:

1. Foreground mode support

   • Added --foreground / -f flag handling in start() function.

2. Improved signal handling in foreground mode

   • Added a trap to correctly relay SIGTERM/SIGINT to both fdbmonitor and its child processes (especially fdbserver).
   • Ensures clean shutdown and avoids orphaned fdbserver processes.

3. Exit code propagation

   • Foreground mode now propagates the exit status of fdbmonitor back to the container runtime via wait + exit $?.
   • Ensures Docker/K8S can correctly detect failure states.

No behavior change for default daemonized startup (non-Docker deployments) and foreground mode is optional and only activated when explicitly requested. So no impact on existing Doris behaviors too.

What problem does this PR solve?

Issue Number: close #xxx

Related PR: #xxx

Problem Summary:

Release note

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Check List (For Author)

• Test

  • [ ] Regression test
  • [ ] Unit Test
  • [ ] Manual test (add detailed scripts or steps below)
  • [ ] No need to test or manual test. Explain why:
    • [ ] This is a refactor/code format and no logic has been changed.
    • [ ] Previous test can cover this change.
    • [ ] No code files have been changed.
    • [ ] Other reason

• Behavior changed:

  • [ ] No.
  • [ ] Yes.

• Does this need documentation?

  • [ ] No.
  • [ ] Yes.

Check List (For Reviewer who merge this PR)

• [ ] Confirm the release note
• [ ] Confirm test cases
• [ ] Confirm document
• [ ] Add branch pick label

[Thearas]

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1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
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4. Which code was refactored and why was this part of the code refactored?
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