fix(agent_engines): delete throwaway sessions for session events

[AnupamKumar-1]

Fixes #7119. async_stream_query(session_events=...) creates a managed session to replay caller-supplied events when no session_id is given. That session's ID was never returned to the caller, so it could not be deleted or have a retention policy applied. It therefore persisted until the default 365-day TTL, including for an empty session_events=[] call.

This marks sessions created for the session_events path as throwaway and deletes them in finally, matching the existing cleanup pattern in the sibling _StreamRunRequest path. Once the session is created, cleanup is guaranteed even if append_event, run config validation, or the runner raises. A failed delete is logged via _warn rather than masking the original exception or blocking _force_flush_otel.

Scope: only the session_events branch is treated as throwaway. A plain call with neither session_id nor session_events still creates and keeps a session, preserving current behavior. The broader case is noted in the issue as a possible follow-up and is not addressed here.

Tests added:

• throwaway session deleted after a normal session_events call
• plain call without session_id still keeps its session
• explicit caller-supplied session_id is never deleted
• throwaway session deleted when the runner raises mid-stream
• throwaway session deleted when append_event raises
• throwaway session deleted on early generator close (aclose())

[AnupamKumar-1]

Hi @matthew29tang, could you please take a look at this PR when convenient for you? Thanks!