feat: Add create_session and get_session classmethods

[ecanlar]

Summary

• Adds create_session(user_id, ...) classmethod to all SQL-based session backends (SQLiteSession, AsyncSQLiteSession, SQLAlchemySession) that creates a new session with an auto-generated UUID session_id, persisting user and session rows immediately
• Adds get_sessions(user_id, ...) classmethod that retrieves all sessions for a user as a list of ready-to-use session instances, with limit/offset pagination support
• No changes to SessionABC or the Session protocol — these methods are backend-specific conveniences

API

# Create a new session (session_id auto-generated)
session = await SQLiteSession.create_session("alice", db_path="./data.db")
print(session.session_id)  # "550e8400-e29b-41d4-..."

# Retrieve all sessions for a user
sessions = await SQLiteSession.get_sessions("alice", db_path="./data.db")
for s in sessions:
    items = await s.get_items()  # each instance is fully functional
    print(s.session_id, len(items))

# With pagination
page = await SQLiteSession.get_sessions("alice", db_path="./data.db", limit=10, offset=0)

Motivation

Builds on #2809 (agent_users table). Currently the caller must invent a session_id and the session row is only created lazily on add_items(). This PR provides a proper factory pattern where:

1. The user_id is the primary entry point (not the session_id)
2. The session_id is generated internally via uuid4()
3. get_sessions returns fully functional session instances (not just IDs like get_sessions_for_user)

Test plan

• All 35 existing session tests pass (backward compatible)
• test_sqlite_create_session — verifies UUID generation, DB persistence, and normal usage
• test_sqlite_get_sessions — verifies retrieval of all user sessions, item preservation, empty result for unknown user
• test_sqlite_get_sessions_pagination — verifies limit/offset pagination
• test_sqlite_create_multiple_sessions_for_user — verifies multiple sessions per user with unique IDs

🤖 Generated with Claude Code

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve SQLiteSession positional argument order

Inserting users_table and user_id before session_settings changes the meaning of existing positional calls. Code that previously did SQLiteSession(session_id, db_path, sessions_table, messages_table, SessionSettings(...)) will now pass the SessionSettings object as users_table, which later produces invalid SQL/table-name behavior at runtime. This is a backward-incompatible API break for existing callers that used positional arguments.

Useful? React with 👍 / 👎.

[chatgpt-codex-connector]

Avoid making existing session backends abstract

Adding create_session/get_session as abstract methods on SessionABC makes every subclass that doesn't implement them uninstantiable. In this commit, backends like OpenAIConversationsSession, RedisSession, DaprSession, and EncryptedSession still do not define these methods, so constructing them will raise TypeError for abstract methods. This is an immediate runtime regression for those session backends.

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[chatgpt-codex-connector]

Migrate preexisting SQLite schemas before writing user_id

This write path now always inserts into (session_id, user_id), but schema setup only uses CREATE TABLE IF NOT EXISTS and does not alter existing agent_sessions tables. For users upgrading with an existing SQLite DB created by earlier versions (without user_id), add_items() will fail with sqlite3.OperationalError: table ... has no column named user_id on first write.

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[seratch]

For the reason I mentioned at #2808 (comment), we don't plan to have this change at least for now.