Feat: Add Optional Structured Session Storage

[habema]

Resolves #1385

This PR introduces an optional structured storage mode to SQLiteSession to improve the observability and query-ability of conversation histories.

When a user initializes a session with structured=True, two new tables are created and populated alongside the existing raw JSON log:

• agent_conversation_messages: Stores distinct user, assistant, and system messages.
• agent_tool_calls: Records tool call invocations and their corresponding outputs.

This makes it significantly easier to analyze and debug agent interactions using standard SQL, addressing the limitations of querying JSON blobs.

Key points:

• The feature is opt-in. Existing implementations are unaffected.
• Foreign key constraints ensure data integrity when items are popped or sessions are cleared.
• Includes comprehensive unit tests and updated documentation.