enhance compile_commands support and add test cases

[luadebug]

#6548

[gemini-code-assist]

Summary of Changes

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This pull request significantly enhances Xmake's compile_commands support by introducing a new policy that allows developers to control its generation on a per-target basis. It also refines a core utility function, os.isexec, for improved cross-platform compatibility and includes new test cases to ensure the reliability of these changes.

Highlights

• New Policy for Compile Commands: Introduced a new policy, build.compile_commands, which defaults to true, allowing granular control over the generation of compile_commands.json for individual targets.
• Conditional Compile Commands Generation: The compile_commands.lua plugin now respects the build.compile_commands policy, enabling users to explicitly disable compile_commands.json output for specific build targets.
• Enhanced os.isexec Robustness: The os.isexec function in xmake/core/base/os.lua has been improved to gracefully handle scenarios where os._access might not be available, ensuring correct behavior by returning true if the file exists.
• Comprehensive Test Cases: Added new test cases under tests/projects/policy/compile_commands to thoroughly validate the functionality of the build.compile_commands policy, including scenarios where it is both enabled and disabled.

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[gemini-code-assist]

Code Review

This pull request introduces a new policy build.compile_commands to selectively disable the generation of compile_commands.json for specific targets. The implementation is sound and is accompanied by a new test case to verify the functionality. I've also noted a small defensive improvement in os.isexec. My only suggestion is a minor cleanup in the new test case to use a newly added source file, which improves clarity. Overall, these are good changes that enhance the flexibility of xmake.