fix(session): drop user registry check for message role_id

[qin-ctx]

Description

Remove the session add_message user-registry validation for role_id so message actor IDs remain backward-compatible and are not forced to map to registered account users.

Related Issue

N/A

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Test update

Changes Made

• removed api_key_manager.has_user(...) validation from session role_id resolution for add_message
• restored compatibility for unregistered/default user contexts when writing user messages
• updated the existing session API test to assert unregistered role_id values are accepted

Testing

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have tested this on the following platforms:
  • Linux
  • macOS
  • Windows

Checklist

• My code follows the project's coding style
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Screenshots (if applicable)

N/A

Additional Notes

session.role_id is now treated purely as the message actor identifier on this HTTP path instead of being constrained by account user registration.

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