fix(usage): reject pre-cancelled quota waits

[outlier27-cell]

Problem

Closes #733. waitForUsage() can return an already-fulfilled credential or quota result after its caller's AbortSignal has already been cancelled. This lets a cancelled /usage collection observe stale data at the shared waiting boundary.

Value

Cancellation now wins before a provider result becomes observable, preserving the caller's lifecycle boundary without taking ownership of Pi's credential-refresh operation.

Approach

Check signal.throwIfAborted() before registering the promise race. Keep the existing in-flight abort listener and cleanup behavior unchanged. Add a deterministic regression that uses a pre-cancelled signal and an already-fulfilled promise, and verifies that the original cancellation Error identity is preserved.

Validation

• node --test --experimental-strip-types tests/extensions/usage/providers.test.ts (13 passed)
• Direct production-helper repro now reports { "returnedResult": false, "keptReason": true }.
• bun run check completed its config, docs, discipline, web, format, lint, and typecheck gates without reported errors.
• bun run test was run after the isolated worktree dependency install; its child processes completed without a reported failure.
• git diff --check

Impact

• User-visible behavior: a cancelled /usage wait no longer emits a result that was already available.
• Model-visible tools and schema: unchanged.
• Runtime lifecycle: only pre-cancelled wait admission changes; in-flight cancellation, provider requests, and cache storage retain their existing owners.
• Persisted data and configuration: unchanged.

[outlier27-cell]

@tt-a1i 请审核此 PR。#733 的最小取消边界修复已在 0cdac2c 完成，完整 exact-head GitHub CI（Node、Windows、UI、E2E）均通过；当前可合并，仅等待审核。

[outlier27-cell]

@testikun @tt-a1i 请审核此 PR。最新提交的 CI 已通过，当前仅等待维护者审核与合并。