[Bug]: Grok threads do not show context-window usage in the composer #8382

Before submitting

• I searched existing issues and did not find an active duplicate.
• I included enough detail to reproduce or investigate the problem.

Related work: the broader #5418 is closed, and #5405 is an open pre-#8358 implementation PR.

Area

apps/server

Steps to reproduce

1. Run a T3 Code build containing fix(grok): improve skills, plans, usage, and turn reliability #8358.
2. Start a Grok thread with a model that advertises _meta.totalContextTokens over ACP.
3. Send a prompt and wait for Grok to report _meta.totalTokens in session/update or the final prompt response.
4. Inspect the context-window meter beside the composer.

Expected behavior

Grok threads should show the existing context-window meter with used tokens, maximum tokens, and fill percentage, matching Codex and other providers. The value should update when Grok reports a newer usage snapshot.

Actual behavior

The meter is absent for Grok threads.

#8358 added Grok token and cost records to the historical Usage page by parsing updates.jsonl. That is a separate pipeline. The live Grok adapter still does not emit thread.token-usage.updated, so the existing ingestion path never creates context-window.updated activity for the composer.

Impact

Minor bug or occasional failure

The failure is consistent, but it does not block a turn. Users cannot see when a long Grok thread is approaching its context limit or decide when to compact or start a new thread.

Version or commit

main after #8358 (ead4ce52a1624d9c55461f61524580978fc8b719)

Environment

Web or desktop client with the Grok provider.

Logs or stack traces

No error is shown. This is a missing provider-runtime event.

Technical notes

• ProviderRuntimeIngestion already maps thread.token-usage.updated to context-window.updated.
• ContextWindowMeter already renders that activity. No new UI component is needed.
• Grok exposes context fill through ACP _meta.totalTokens and model capacity through availableModels[]._meta.totalContextTokens. A standard ACP usage_update should also be accepted when available.
• fix(grok): surface context window usage in the composer #5405 demonstrates the intended behavior and contains focused tests, but it predates fix(grok): improve skills, plans, usage, and turn reliability #8358 and should be rebased or manually ported rather than merged unchanged.
• Keep the change on top of fix(grok): improve skills, plans, usage, and turn reliability #8358's current AcpRuntimeModel, GrokAcpSupport, and GrokAdapter structure.

Acceptance criteria

• A usage-bearing Grok notification or completed prompt emits a positive thread.token-usage.updated snapshot.
• The snapshot includes the active model's context-window maximum when Grok advertises it.
• The existing composer meter appears and updates for Grok threads.
• Interrupted turns do not overwrite the last valid context snapshot.
• Focused adapter and ingestion tests prove the public event and visible activity contract.

Workaround

There is no in-app workaround. Users can inspect Grok directly or start a new thread conservatively.