Infer MCP sessions from a sliding window

What does this MR do and why?

Infer MCP sessions from a sliding window

MCP is stateless - session ids were removed from the specification in the 2026-07-28 revision - so there is nothing on a tools/call request to group calls by. A session is inferred from caller identity plus a 15 minute idle window capped at 30 minutes, held in Redis, and recorded as an ai_governance_sessions row.

The window is evaluated in Ruby rather than left to the Redis TTL, so a key outliving its window cannot silently extend a session.

The OAuth application behind the request is the only agent identity that survives statelessness. It is client-declared and never verified, so it is mapped onto a known slug and is attribution only, never authorization.

Nothing calls this yet.

Changelog: added EE: true

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##Queryies -- create_session INSERT INTO ai_governance_sessions (namespace_id, project_id, user_id, external_xid, source, status, session_started_at, agent_type, created_at, updated_at) VALUES ($1, $2, $3, $4, 3, 1, $5, $6, $7, $8) RETURNING id

-- resume / resume_after_lost_claim SELECT "ai_governance_sessions".* FROM "ai_governance_sessions" WHERE "ai_governance_sessions"."id" = $1 LIMIT 1 -- Index Scan using ai_governance_sessions_pkey (primary key lookup)

-- mint, lost-claim path DELETE FROM ai_governance_sessions WHERE id = $1 -- Delete via primary key

All three are single-row primary-key operations. The INSERT checks two partial unique indexes, both namespace_id-led: i_ai_governance_sessions_on_namespace_source_external_xid and i_ai_governance_sessions_on_namespace_workflow