docs(openapi): improve docs for session model and default session endpoints

[akshay-kumar-bm]

Description

This PR improves the generated OpenAPI/Swagger documentation for session
management by adding clearer endpoint metadata and richer schema docs
(descriptions + examples).

What changed

• Session model docs: add field descriptions/examples for Session so the API
  schema is self-explanatory.
• Default session endpoints: improve OpenAPI docs for the built-in session
  management routes (create/get/list/update/delete sessions + events/artifacts
  where applicable) so request/response shapes and common errors are clearer.
• System endpoints: enhance docs for basic endpoints like /health, / version,
  and /list-apps (where present in this server), including clearer summaries
  and error notes.

Why

The OpenAPI output was correct but lacked enough context (field meanings,
examples, and expected error cases) for users reading Swagger UI or generating
clients.

Testing

Documentation-only change (OpenAPI metadata / schema docs).

• Unit tests: Not run.
• Manual check: Confirm Swagger UI shows the new descriptions/examples for
  session-related schemas and endpoints.

Notes

• Includes an empty commit (chore: rerun CI) to re-trigger GitHub Actions
  without changing code.

Checklist

• Read CONTRIBUTING.md.
• Self-reviewed the change.
• Added/updated unit tests (N/A for doc-only change).

1. • Ran unit tests locally (not run for doc-only change).

[akshay-kumar-bm]

Here some screenshort compare origin and updateing doc