fix(grok): Complete turns from xAI prompt completion

[mwolson]

Summary

• Add an xAI prompt-complete fallback for Grok ACP prompts that finish on the wire but leave session/prompt stranded.
• Keep Grok adapter session snapshots in running only while prompts are actually in flight, then clear activeTurnId on completion.
• Add direct runtime and adapter regression coverage for the Grok completion hang, plus protocol coverage for Grok extension notifications batched before a standard prompt response.

Problem and Fix

This is a narrow bridge fix for the current Grok Composer 2.5 Fast hang while the sturdier ACP/orchestrator work in #2829 continues.

Problem and Why it Happened	Fix
Grok can emit _x.ai/session/prompt_complete and return the session to idle while the standard session/prompt RPC remains stranded. T3 Code waits for session/prompt before emitting turn.completed, so the composer can stay stuck on Stop even though the turn is done.	Register a per-session fallback before each ACP prompt and race the standard prompt RPC against _x.ai/session/prompt_complete, synthesizing the normal ACP PromptResponse shape when the xAI notification wins.
Grok adapter session snapshots could retain activeTurnId after the prompt settled. Consumers reading adapter session state could still see the session as active.	Mark Grok sessions running while prompt work is active and restore ready while clearing activeTurnId when the final prompt in the turn settles.
The existing tests did not reproduce a Grok prompt that completes through xAI's private notification while the prompt RPC never returns.	Extend the ACP mock agent with a prompt-complete-then-hang mode and cover it at both AcpSessionRuntime and GrokAdapter levels.

Defensive Fixes

Problem and Why it Happened	Fix
Grok extension notifications can be batched before the normal session/prompt response.	Add effect-acp client coverage that keeps the standard prompt response routable after Grok extension notifications in the same input batch.
Multiple pending prompt fallbacks on one session are unlikely in the current composer flow but possible through steering paths.	Resolve fallback completions FIFO by session, which avoids letting an older completion unblock the newest prompt.

Validation

• vp test apps/server/src/provider/Layers/GrokAdapter.test.ts apps/server/src/provider/acp/AcpJsonRpcConnection.test.ts packages/effect-acp/src/client.test.ts
• vp run typecheck
• vp check

Note

Fix Grok turn completion by resolving prompts from xAI _x.ai/session/prompt_complete notifications

• AcpSessionRuntime now races the standard session/prompt RPC against a fallback Deferred resolved by _x.ai/session/prompt_complete notifications, handling the case where Grok emits its own completion signal but never settles the RPC.
• Stale prompt_complete notifications (for already-completed prompts) are tracked and ignored using a bounded list of completed prompt IDs.
• GrokAdapter marks sessions as running at prompt start and transitions back to ready only when the last in-flight prompt for the active turn settles, including on error or RPC hang.
• A mock agent script gains environment flags to simulate xAI prompt completion then hang, stale completions, and prompt failures for integration testing.

^{Macroscope summarized 1b2c893.}

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Note

Medium Risk
Changes Grok turn completion and session lifecycle in the provider stack; behavior is narrow and heavily regression-tested but affects when turn.completed fires and adapter session snapshots update.

Overview
Fixes Grok turns that finish on the wire but never unblock the UI because session/prompt never returns.

AcpSessionRuntime now races each prompt against Grok’s _x.ai/session/prompt_complete notification, building a normal PromptResponse when the xAI signal wins first. Already-finished promptId values are remembered so late duplicate completions are ignored.

GrokAdapter sets sessions to running while prompts are in flight and returns them to ready (clearing activeTurnId and emitting turn.completed) only when the last prompt for the active turn settles, including on RPC failure via settlePromptInFlight.

The ACP mock agent gains env-driven modes (prompt-complete-then-hang, stale completion, forced prompt failure), with coverage in AcpJsonRpcConnection, GrokAdapter, and effect-acp client batching tests.

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Approvability

Verdict: Needs human review

1 blocking correctness issue found. This PR introduces a new fallback mechanism for completing Grok turns with complex async state management. Unresolved review comments identify potential race conditions where turn.completed may not be emitted in concurrent prompt scenarios, warranting human review to verify correctness.

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