Resolve #2868: Cover async rejection cleanup in raceWithAbort

[ayden94]

Summary

Add the missing asynchronous-rejection regression coverage for the documented raceWithAbort() listener cleanup contract.

Linked context: Closes #2868

Changes

• add a deterministic microtask-delayed rejection case
• assert raceWithAbort() preserves the original rejection object
• assert the exact race-owned abort listener is removed after rejection

Testing

• pnpm vitest run packages/runtime/src/abort.test.ts (10 passed)
• pnpm --filter @fluojs/runtime typecheck
• pnpm --filter @fluojs/runtime test (27 files, 301 passed)
• changed-file LSP diagnostics: clean
• full pnpm verify passed build/typecheck/lint; two unrelated parallel-suite failures passed when rerun in isolation, so canonical GitHub CI is the merge gate

Release impact

• This PR has consumer-visible release impact and includes a changeset.
• This PR has no consumer-visible release impact.

Test-only coverage of existing behavior; no Changeset is required.

Public export documentation

• No public exports changed.
• Existing source and README documentation remains unchanged.

Behavioral contract

• No documented behavioral contracts were removed without migration notes.
• No new behavioral contract was introduced.
• The existing all-settlement listener cleanup invariant now covers asynchronous rejection directly.

Platform consistency governance (SSOT)

• No platform contract docs or governance SSOT changed.
• The change is isolated to the canonical runtime abort unit test.