feat(acp): update schema to 1.12.0 and align v2 lifecycle

[benbrandt]

Summary

• Upgrade the pinned schema crate and lockfile from 1.11.0 to 1.12.0.
• Forward unstable_session_system_prompt, include it in the unstable aggregate, and document capability-gated system prompts through the existing complete-request session helpers.
• Send a current-state snapshot after replay and before every successful v2 resume response in Testy and the echo agent, including when replay is omitted.
• Handle foreground work starting in requires_action and ending without running in the one-shot client and cookbook.
• Update the book for metadata patches, aggregate foreground states, cancellation and close ordering, insertion receipts, authentication failures, replay IDs/cursors, experimental feature negotiation, absolute Git patch paths, and upstream NES removal.
• Add regressions for system-prompt typed decoding, metadata key patches, blocked-first foreground work, and resume snapshot ordering.

Validation

• just test: 895 passed, 45 ignored, including workspace binaries/examples and doctests.
• cargo check -p agent-client-protocol --no-default-features --features unstable_session_system_prompt --locked
• cargo check -p agent-client-protocol --no-default-features --features unstable_protocol_v2,unstable_session_system_prompt --locked
• cargo fmt --all -- --check
• mdbook build (after installing the ignored Mermaid assets)
• Ran the compiled v2_one_shot_client against simple_agent_v2 over stdio; insertion, output, idle completion, and close succeeded.

Notes

The initial full test run hit acp_agent::tests::protocol_eof_still_reports_nonzero_child_exit; it passed on subsequent full runs. No unrelated child-process behavior changes are included. SDK package versions remain managed by the release workflow.