Grok/ACP: sessionUpdate parity for live Grok agent stdio

[EnzoTironi]

Before submitting

• I searched existing issues and did not find a duplicate.
• I am describing a concrete problem or use case, not just a vague idea.

Area

apps/server

Problem or use case

Live Grok (grok agent stdio) emits ACP session/update kinds and xAI extension hooks that the shared AcpRuntimeModel does not fully model. Adapters either drop them or cast ad hoc.

Proposed solution

Extend shared ACP parsing (AcpRuntimeModel / AcpCoreRuntimeEvents) for: agent_thought_chunk, usage_update, available_commands_update, config_option_update, session_info_update, user_message_chunk, unknown fallback, and xAI exit_plan_mode helpers.

Why this matters

Without shared parser coverage, every ACP provider invents its own casting. Grok (and Cursor) cannot surface reasoning, usage, or commands reliably.

Smallest useful scope

Parser + mock-agent fixtures only. No Grok product brand, no UI.

Alternatives considered

Per-adapter casts in GrokAdapter only — works short-term but breaks shared Cursor path and exhaustiveness.

Risks or tradeoffs

Shared streaming path; must stay additive and forward-compatible via unknown updates.

Examples or references

PR #5422. Related open usage work: #5405.