Validate pilot registration fields against the CS

[aldbr]

Context

POST /api/pilots/ (added in #570) accepts grid_type, grid_site and destination_site as free strings, and PATCH /api/pilots/metadata similarly accepts GridType, GridSite, DestinationSite and Queue without validation. A typo silently creates pilots attached to nonexistent sites, which then pollutes accounting and monitoring groupings.

The vo field is already validated against the registry (400 for unknown VOs) since #570.

Legacy DIRAC's PilotManagerHandler.addPilotReferences performs no validation either, so this is an improvement over parity, not a regression fix.

Blocker

The site list cannot be validated today: the diracx Config schema does not model the Resources/Sites section of the CS. ResourcesConfig currently only contains Computing.OSCompatibility, with extra="ignore" and its own TODO ("Remove this once the model is extended to support everything", diracx-core/src/diracx/core/config/schema.py).

DoD

• Model Resources/Sites in the Config schema (with DIRAC_COMPAT_ENABLE_CS_CONVERSION support for the legacy CFG format, as done for OSCompatibility).
• Validate grid_site / destination_site / grid_type against the modeled site list on POST /api/pilots/ and on PATCH /api/pilots/metadata (GridSite / DestinationSite fields). Same question applies to Queue.
• Decide the failure mode: reject with 400, or accept-with-warning for pilots from sites not (yet) present in the CS (relevant for vacuum pilots appearing at new sites).