fix(python): populate usage_info when resuming session

[Coderxrohan]

Issue

When resuming a session, session.usage_info is not populated until a prompt is sent, making it impossible to access token usage during the initial resumed session phase.

Fix

Populate usage_info directly from the session.resume response and attach it to the CopilotSession object during resume.

Impact

Token usage is now immediately available after resume_session, enabling clients to display current usage without waiting for the first prompt.

Testing

• Resumed an existing session locally
• Verified session.usage_info is available immediately after resume

Fixes: #312

[Coderxrohan]

Added usage_info as a first-class property on CopilotSession, populated it during both create_session and resume_session, and kept it in sync by updating it on incoming session events. This makes token usage available immediately after resuming a session without waiting for the first prompt.