feat(api): abort signal core plumbing — interface, Task.ts, single-completion-handler #615

Parent issue

Part of #404 — Stop does not abort underlying HTTP request

Scope

Core plumbing that all providers depend on. Must land first.

Changes needed

1. src/api/index.ts — Add abortSignal?: AbortSignal to ApiHandlerCreateMessageMetadata (already done in feat(api): pass abortSignal to streaming API calls for all providers (#404) #434)
2. src/api/index.ts — Update SingleCompletionHandler.completePrompt signature to accept metadata?: ApiHandlerCreateMessageMetadata as a second parameter. Without this, all completePrompt abort work is dead code (review finding #1)
3. src/core/task/Task.ts — Wire currentRequestAbortController.signal into metadata.abortSignal (already done in feat(api): pass abortSignal to streaming API calls for all providers (#404) #434, but prefer including it in the object literal at construction time rather than post-mutation)
4. src/utils/single-completion-handler.ts — Forward metadata to handler.completePrompt(promptText, metadata)

Tests

• src/core/__tests__/task-abort-signal-passing.spec.ts — add signal identity assertion (toBe not just toBeInstanceOf) and a sequential-requests test confirming fresh controllers

Acceptance criteria

• SingleCompletionHandler.completePrompt interface accepts optional metadata
• single-completion-handler.ts forwards metadata to the provider
• Task.ts attaches abortSignal to metadata
• Tests verify signal identity and fresh-controller-per-request