feat: make bthread stack trace signal number configurable

[ZhengweiZhu]

What problem does this PR solve?

Issue Number: resolve #3118

Problem Summary:

What is changed and the side effects?

Changed:

Side effects:

• Performance effects:
  NO
• Breaking backward compatibility:
  NO

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Check List:

• Please make sure your changes are compilable.
• When providing us with a new feature, it is best to add related tests.
• Please follow Contributor Covenant Code of Conduct.

[ZhengweiZhu]

@chenBright Take a look~

[Copilot]

Pull Request Overview

This PR makes the signal number used for bthread stack tracing configurable instead of hardcoded to SIGURG. This addresses compatibility issues where SIGURG may already be registered by other libraries like cgo.

Key Changes:

• Introduces a new flag signal_number_for_trace to configure the signal number (defaults to SIGURG for backward compatibility)
• Updates TaskTracer to use the configured signal number instead of hardcoded SIGURG
• Adds test coverage to verify the configuration works with alternative signals

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/bthread/task_tracer.cpp	Adds the configurable flag and updates signal registration/sending to use the configured signal number
src/bthread/task_tracer.h	Adds getter for signal number and stores it as instance variable; changes RegisterSignalHandler to non-static
src/bthread/task_control.cpp	Updates worker interruption to use the configured signal number instead of hardcoded SIGURG
test/brpc_builtin_service_unittest.cpp	Adds test initialization to verify configuration with SIGUSR2

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[Copilot]

The method signature changed from static to non-static, which is a significant API change affecting how this method is called. This change should be documented with a comment explaining why it needs to be non-static (i.e., to access the instance variable _signal_num).

Suggested change

	// This method must be non-static in order to access the instance variable _signal_num.

[ZhengweiZhu]

ping~

[wwbmmm]

LGTM

[chenBright]

LGTM