refactor: modernize session management and token validation

[solracsf]

This PR is a comprehensive audit and refactor, resolving bugs and performance problems identified through iterative code review. Refactor user session management and token validation logic for improved clarity and functionality. Introduce constants for session data and enhance error handling.

• Catch exceptions when instantiating bearer token validators, so a missing or broken DI entry no longer crashes the authentication chain.
• Catch exceptions during provisioning strategy resolution and execution, and log the failure instead of aborting login.
• Tighten bearer token parsing and basic UID validation before using the resolved user ID.
• Treat duplicate validations for the same (provider, userId) as valid, while still rejecting genuinely ambiguous matches across different providers or user IDs.
• Pre-instantiate validators outside provider loop

[julien-nc]

Thanks a lot for refactoring this. Could you add a doc block comment for all the new methods with a global description? Especially if they change the overall behaviour of the backend.

[solracsf]

@julien-nc will adress all your comments at once ASAP.

[julien-nc]

@solracsf There is no rush 😁