feat(server): ACP sessionUpdate parity for Grok clients

[EnzoTironi]

What Changed

Shared ACP session/update parsing so live Grok (and other ACP clients) map into typed runtime events instead of being dropped:

• agent_thought_chunk → reasoning stream (streamKind: reasoning_text)
• usage_update → thread.token-usage.updated via makeAcpTokenUsageEvent
• available_commands_update, config_option_update, session_info_update, user_message_chunk
• UnknownSessionUpdate fallback for forward compatibility
• session/set_model optional _meta pass-through (needed by Grok effort)
• acp-mock-agent toggles for Grok-shaped session behavior

Fixes #5417

Why

Live grok agent stdio already emits these update kinds. Without shared parser coverage, each provider adapter drops them or casts ad hoc. Putting the model in AcpRuntimeModel / AcpCoreRuntimeEvents keeps orchestration pure.

UI Changes

None (server/shared ACP only).

Checklist

• This PR is small and focused (~+379 / 6 files)
• I explained what changed and why
• UI screenshots N/A
• Video N/A

Test plan

• AcpRuntimeModel / AcpCoreRuntimeEvents unit tests
• Live Grok 0.2.x initialize/session on macOS

Model: grok-4.5 (Grok Build)

---

Note

Medium Risk
Touches shared ACP parsing and runtime events used by multiple provider adapters; behavior is covered by unit tests and limited adapter wiring, but incorrect mapping could affect token usage and streaming UI.

Overview
Brings shared ACP session/update handling in line with what live Grok already emits, so updates are parsed into typed events instead of being dropped.

AcpRuntimeModel now recognizes agent_thought_chunk (reasoning via streamKind: reasoning_text), usage_update, available_commands_update, config_option_update, session_info_update, user_message_chunk, and an UnknownSessionUpdate fallback. makeAcpTokenUsageEvent maps usage into thread.token-usage.updated, and makeAcpContentDeltaEvent honors an optional streamKind.

CursorAdapter forwards UsageUpdated and reasoning ContentDelta into those helpers. AcpSessionRuntime exposes authenticateResult on start and passes optional _meta through session/set_model (e.g. Grok reasoning effort).

acp-mock-agent gains Grok-shaped toggles: /compact → auto_compact_completed, prompt _meta token usage, init availableCommands, unsolicited JSON-RPC responses, and session_info_update.

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Note

Add ACP sessionUpdate parity for Grok clients including usage, reasoning, and session info events

• Extends parseSessionUpdateEvent in AcpRuntimeModel.ts to parse additional ACP session events: agent_thought_chunk (reasoning text), usage_update, available_commands_update, session_info_update, user_message_chunk, and unknown updates. All events now include rawPayload.
• Adds makeAcpTokenUsageEvent factory in AcpCoreRuntimeEvents.ts to emit thread.token-usage.updated events with token breakdown fields; extends makeAcpContentDeltaEvent with an optional streamKind (assistant_text | reasoning_text).
• Updates CursorAdapter.ts to handle UsageUpdated events by publishing thread.token-usage.updated and to pass streamKind/rawPayload through ContentDelta events.
• Exposes authenticateResult from AcpSessionRuntime.start() and adds _meta passthrough to setSessionModel.
• Updates the ACP mock agent script to handle /compact prompts, emit optional session_info_update, and return usage metadata on prompt responses.

^{Macroscope summarized a45fb3d.}

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[macroscopeapp]

Approvability

Verdict: Needs human review

This PR adds new feature capability for handling Grok client session updates, introducing multiple new event types (usage, commands, config options, session info, reasoning tokens) and their parsing/runtime integration. New features that add runtime behavior warrant human review.

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Cursor Bugbot has reviewed your changes using high effort and found 1 potential issue.

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[EnzoTironi]

Head fork was deleted and GitHub closed this PR permanently (cannot reopen).

Correct recovery PR: pull request title must not be blank

Same commits/branch tip as this one. #5424 and #5425 stay closed on purpose (absorbed into the Grok native parity PR).

[EnzoTironi]

Correction — recovery PR is: #5502