Add long-running chat run controls

[chubes4]

Summary

• Adds backend-neutral runCapabilities primitives for optional active-run cancellation and queued follow-up messages.
• Exposes activeRunId, getRunId, cancelRun, canCancelRun, and canQueueMessage through useChat so adapters can wire their own backend contract.
• Updates ChatInput/message rendering to show a stop control and optimistic queued/failed message states only when the capability is supplied.

Notes

• Preserves existing disabled-input behavior by default when no queue capability is configured.
• Coordinates with the contract shape discussed in Add canonical chat run control: run IDs, status, cancellation, and queued messages Automattic/agents-api#236 without hardcoding Agents API, WordPress, Studio Web, or frontend-agent-chat details.
• Closes Add generic Stop and Queue UX for long-running chat turns #38.

Verification

• npm run build
• homeboy test --path /Users/chubes/Developer/chat@feat-long-running-turn-primitives --extension nodejs (0 tests detected by repo)
• homeboy lint --path /Users/chubes/Developer/chat@feat-long-running-turn-primitives --extension nodejs (no lint surface detected)
• git diff --check

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Implementing the backend-neutral run-control API, UI state wiring, documentation, and verification. Chris remains responsible for review and merge.

[chubes4]

Follow-up alignment after Automattic/agents-api#237 merged:

• Kept this package backend-neutral while making runId first-class on SendResult / ContinueResult from generic run_id wire fields.
• Added generic ChatRun, ChatRunStatus, and QueueMessageResult types using camelCase adapter-facing names.
• Passes the active runId into onQueueMessage so adapters can map cleanly to their backend queue contract.
• Allows onQueueMessage to return { queuedMessageId, position, runId, sessionId, status } for adapters that receive queued-message acknowledgement payloads.

Verification after this update:

• npm run build
• homeboy test --path /Users/chubes/Developer/chat@feat-long-running-turn-primitives --extension nodejs (0 tests detected by repo)
• homeboy lint --path /Users/chubes/Developer/chat@feat-long-running-turn-primitives --extension nodejs (no lint surface detected)
• git diff --check