chore(api-spec): update session endpoint specs

[bjoern-m]

Description

This PR updates the OpenAPI specification for the session endpoints to align the response definitions to the API changes made in teamhanko/hanko#2003 .

Changes Made

1. Endpoint Response Updates:

   • Added 500 responses (InternalServerError) to the GET /sessions/validate and POST /sessions/validate endpoints for better documentation of potential server errors.
   • Enhanced the 400 responses (BadRequest) for these endpoints.

2. Schema Enhancements:

   • Introduced a new schema, JWTClaims, to describe the claims extracted from JWTs.
   • Includes fields are: subject, issued_at, expiration, audience, issuer, email, and session_id.
   • Deprecated the following fields in the ValidateSessionResponse schema:
     • expires_at
     • user_id

Notes

• Consumers of the API are encouraged to transition to the new JWTClaims schema as the deprecated fields will be removed in a future release.