fix(agnocastlib): use dedicated executor thread for clock

[Koichi98]

Description

Add use_clock_thread option to NodeTimeSource to process /clock subscription on a dedicated executor thread, aligning with rclcpp's TimeSource implementation.

Changes:

• Add use_clock_thread parameter (default: true)
• Create dedicated AgnocastOnlySingleThreadedExecutor and thread for clock callbacks
• Add proper cleanup in destroy_clock_sub()

Related links

https://github.com/ros2/rclcpp/blob/rolling/rclcpp/src/rclcpp/time_source.cpp

How was this PR tested?

• Autoware (required)
• bash scripts/e2e_test_1to1 (required)
• bash scripts/e2e_test_2to2 (required)
• kunit tests (required when modifying the kernel module)
• sample application

Notes for reviewers

Version Update Label (Required)

Please add exactly one of the following labels to this PR:

• need-major-update: User API breaking changes
• need-minor-update: Internal API breaking changes (heaphook/kmod/agnocastlib compatibility)
• need-patch-update: Bug fixes and other changes

Important notes:

• If you need need-major-update or need-minor-update, please include this in the PR title as well.
  • Example: fix(foo)[needs major version update]: bar or feat(baz)[needs minor version update]: qux
• After receiving approval from reviewers, add the run-build-test label. The PR can only be merged after the build tests pass.

See CONTRIBUTING.md for detailed versioning rules.

[Copilot]

Pull request overview

This PR adds support for a dedicated executor thread for handling clock messages in the agnocastlib NodeTimeSource implementation. The change introduces a new optional parameter use_clock_thread (defaulting to true) that creates a separate thread with its own single-threaded executor to process /clock topic messages. This prevents clock callbacks from blocking or being delayed by other node operations.

Changes:

• Added use_clock_thread parameter to NodeTimeSource constructor to enable/disable dedicated clock thread
• Created dedicated callback group, executor, and thread for clock subscription when enabled
• Added proper thread cleanup in destroy_clock_sub() method

Reviewed changes

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

File	Description
src/agnocastlib/include/agnocast/node/node_interfaces/node_time_source.hpp	Added member variables for clock executor thread, callback group, and use_clock_thread flag; added use_clock_thread parameter to constructor
src/agnocastlib/src/node/node_interfaces/node_time_source.cpp	Implemented dedicated executor thread creation and cleanup logic; added callback group configuration for clock subscription

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