add optional fd leak check

[Guest0x0]

This PR introduces an optional fd leak check, which can be enabled by setting the MOONBIT_ASYNC_CHECK_FD_LEAK. When enabled, the moonbitlang/async event loop will abort if there are unclosed file descriptor managed by the event loop on exit. Currently not much information can be reported on the exact source of the leaked fd, merely the presence of such leak can be known.

To make the scope of the leak check wilder, some internal refactor is performed. Most raw fd usage outside moonbitlang/async/internal/event_loop are eliminated in favor of using @event_loop.IoHandle. is_async=false can be used to disable event loop integration of the fd, but still allowing it to get managed by the library, and hence leak-checked. The semantic of @event_loop.IoHandle::from_fd is also refined a bit. It now automatically closes the input fd on failure, this allows eliminating some error handling boilerplate around the codebase when constructing IO handle.

Several leaks in the existing codebase are detected by the leak check. They are also fixed in this PR.

[coveralls]

Coverage Report for CI Build 307

Coverage increased (+0.6%) to 79.215%

Details

• Coverage increased (+0.6%) from the base build.
• Patch coverage: 14 uncovered changes across 5 files (49 of 63 lines covered, 77.78%).
• No coverage regressions found.

Uncovered Changes

File	Changed	Covered	%
src/internal/event_loop/event_loop.mbt	10	5	50.0%
src/socket/tcp.mbt	8	5	62.5%
src/fs/dir.mbt	8	6	75.0%
src/internal/event_loop/io_unix.mbt	4	2	50.0%
src/internal/event_loop/io_windows.mbt	4	2	50.0%

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	4181
Covered Lines:	3312
Line Coverage:	79.22%
Coverage Strength:	59720.74 hits per line

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💛 - Coveralls