fix(agent-core-v2): observe aborted source rejections

[morluto]

Related Issue

No linked issue. This is a focused fix for deterministic promise ownership behavior in the internal v2 engine.

Problem

When abortable() receives an already-aborted signal, it rejects its returned promise before attaching handlers to the supplied source promise. If that source rejects later, Node emits unhandledRejection; with the default throw policy, that can terminate the process.

The MCP reconnect path can create reconnect work before wrapping it with the tool execution signal, making it the clearest affected caller.

already-aborted signal ──> caller receives AbortError
                                  │
source work continues ────────────┴──> later rejection
                                          │
                              before: unhandled
                              after:  observed by abortable

What changed

• Attach source settlement handlers before checking whether cancellation has already won.
• Remove the abort listener on either source settlement path without creating a detached cleanup promise.
• Add a regression test that observes the actual Node unhandledRejection boundary for the supplied source promise.

This does not attempt to cancel the source operation; it preserves the existing contract while retaining ownership of its eventual rejection.

Test plan

• pnpm --filter @moonshot-ai/agent-core-v2 exec vitest run test/_base/utils/abort.test.ts --reporter=verbose — 9 passed
• pnpm --filter @moonshot-ai/agent-core-v2 test — 4,082 passed
• pnpm --filter @moonshot-ai/agent-core-v2 typecheck — passed
• pnpm --filter @moonshot-ai/agent-core-v2 lint:domain — passed for 938 files
• pnpm exec oxlint --type-aware packages/agent-core-v2/src/_base/utils/abort.ts packages/agent-core-v2/test/_base/utils/abort.test.ts — 0 warnings, 0 errors

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue, or explained the problem above.
• I have added tests that prove the fix works.
• Ran gen-changesets; no changeset is needed for an internal-only agent-core-v2 change.
• No user documentation update is needed for this internal utility fix.

[changeset-bot]

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Latest commit: 338d4bf

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