docs: Clarify required format for thought field

[sruthigandhi]

Was testing the environment and ran into validation errors on /step
that were super confusing. I think thought field has a validator
that requires both 'Observation:' and 'Plan:' sections, but this isn't
in the schema or error messages.

I think the fix is to update the Field description to make the requirement
explicit with an example. That way when someone queries /schema or looks
at the docs, they'll see what format is expected.

Changes:

• Updated thought field description in models.py to document the required format
• Added an example so users know what a valid thought looks like

I tested it on /schema endpoint and the new description shows up correctly.

Let me know if this is the right approach or if there's a better way to handle it.