feat(agents): capture a terminal session's tenant at creation — it cannot be recovered afterwards #16975

Part of #16946. Found while building presence (#16947, PR #16965).

Finding

A terminal session has no tenant, and none can be recovered after the fact. So presence reports every session as UNKNOWN_TENANT — correctly fail-closed, but it means no session can be discovered or addressed within its own tenant.

The route that looked like it might work does not exist:

• AgentTerminalSession (services/agent_terminal/models.py) has no tenant, company, org or owner field.
• conversation_id is used only for log linkage, everywhere it appears (session_manager.py:88, 181, 210, 244).
• The chat-history session and conversation stores composed by ChatHistoryManager carry no user or tenant field.
• The authenticated creator's owner is passed through SessionManager.create_session → _setup_pty_for_session → _register_pty_with_terminal_manager, but only for PTY registration, and it is never persisted on the session.

No username → organisation lookup exists either, so that may need building.

Fix

Capture the tenant at creation, while the authenticated owner is still in hand:

1. Resolve the creating principal's organisation when the session is created.
2. Persist it on AgentTerminalSession as an explicit field.
3. Have the presence feed report it, falling back to UNKNOWN_TENANT only when it genuinely could not be determined.

Acceptance criteria

• A newly created session carries the tenant of the principal who created it
• The tenant is resolved from the authenticated principal, never from a caller-supplied value
• Presence reports a session under its real tenant, and a session whose tenant cannot be determined stays UNKNOWN_TENANT
• A test showing a session is discoverable within its own tenant and invisible to another
• Sessions that already exist are handled explicitly — left UNKNOWN_TENANT, not guessed