feat: expose typed session failures for AIR

[nikita-ashihmin]

Summary

• Negotiate the JetBrains AIR sessionFailure extension when the client advertises an integer version >= 1 and the capability name.
• Return turn-terminal failures only on PromptResponse._meta; publish late, idle, and otherwise session-scoped failures through session/update.
• Keep legacy ACP cancellation and RequestError behavior for capability-off clients.
• Emit bounded fixed safe messages without provider details, stderr, paths, headers, tokens, or stack traces.

Recovery contract

Failures use _meta.jetbrains.air.sessionFailure with a stable id, monotonic revision, phase, category, source, safe message, retryability, and optional real turn attribution. Turn failures use the completed app-server turn id. Session-scoped ids include a per-server epoch so an AIR replay tombstone cannot suppress a new outage after process restart.

AIR owns localized action presentation. Codex supplies typed category and retryability facts. A successful prompt clears only the exact failure revision that was active when that recovery attempt started; a newer failure is never cleared accidentally.

The event routing covers the boundaries that previously lost or misattributed errors: before turn start, after turn/completed, during plan approval, between approval and implementation turnStart, during implementation, after prompt return, process exit, and local slash commands that complete without creating a turn.

Validation

• 394 passed, 28 skipped.
• Build passed.
• Typecheck passed.
• Diff check passed.
• In-memory NDJSON tests cover initialize negotiation, terminal PromptResponse metadata, session-scoped updates, process exit, capability-off behavior, restart-safe ids, and plan/slash-command races.