fix(advanced-sqlite-session): cleanup branch-only messages on delete_branch

[LeSingh1]

Fixes #3346.

AdvancedSQLiteSession.delete_branch deletes the matching turn_usage and message_structure rows but never touched agent_messages. When messages were only referenced by the deleted branch, their agent_messages rows survived the deletion. After the delete:

• branch queries via get_items() correctly omit them (no message_structure row), and
• the rows are still sitting in the base table.

For branch-heavy sessions that meant invisible orphans accumulating.

Repro from the issue:

import asyncio
from agents.extensions.memory import AdvancedSQLiteSession

async def main():
    session = AdvancedSQLiteSession(session_id="repro", create_tables=True)
    await session.add_items([
        {"role": "user", "content": "main question"},
        {"role": "assistant", "content": "main answer"},
    ])
    await session.create_branch_from_turn(1, "branch_only")
    await session.add_items([
        {"role": "user", "content": "branch-only question"},
        {"role": "assistant", "content": "branch-only answer"},
    ])
    await session.delete_branch("branch_only", force=True)

    with session._locked_connection() as conn:
        rows = conn.execute(
            "SELECT id, message_data FROM agent_messages WHERE session_id = ? ORDER BY id",
            (session.session_id,),
        ).fetchall()
    print(rows)
    session.close()

asyncio.run(main())

Before: four rows (the branch-only pair is still there). After this PR: two rows (only main).

The existing _cleanup_orphaned_messages_sync helper already does exactly the right thing — it scopes to self.session_id and removes only rows with no message_structure reference, so it preserves messages that other branches still share via _copy_messages_to_new_branch. The fix calls it inside delete_branch's existing transaction, after the message_structure delete, so the cleanup commits with the rest of the deletion.

Tests:

• test_delete_branch_cleans_branch_only_messages reproduces the issue and asserts only the main messages remain after the branch is deleted.
• test_delete_branch_preserves_messages_shared_with_main forks a branch at turn 2, deletes it, and confirms turn 1's messages stay because main still references them.

Verification: uv run pytest tests/extensions/memory/test_advanced_sqlite_session.py → 38 passed. make format / make lint / make typecheck clean on the touched files.