fix(async): harden async module and add tests

[dlyongemallo]

No description provided.

[Copilot]

Pull request overview

This PR strengthens diffview.async’s core scheduling utilities and adds functional tests that exercise common guard patterns used to abort in-flight async work.

Changes:

• Add functional specs covering control.Signal semantics and a “closing signal” guard pattern.
• Add functional specs validating debounce.debounce_trailing behavior under rapid calls.
• Harden async.timeout and async.scheduler by using explicit callback-parameter counts and handling timer allocation failure.

Reviewed changes

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

File	Description
lua/diffview/tests/functional/async_guards_spec.lua	Adds functional coverage for Signal behavior, closing-guard abort patterns, and trailing debounce behavior.
lua/diffview/async.lua	Adjusts timeout/scheduler wrapping (explicit param count) and adds a fallback path when uv.new_timer() can’t be created.

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