Read adapter: diracx RSS calls + response translation

[aldbr]

Description

Build a self-contained read adapter module that calls the diracx RSS API (phase 1 routes) and returns data in the legacy format expected by the ResourceStatus and SiteStatus singletons' cache refresh methods.

The module owns two responsibilities:

1. Client calls to the diracx RSS endpoints using the generated diracx client, including iterating over all configured VOs and merging results to match the current vO="all" behavior.
2. Response translation from diracx AllowedStatus / BannedStatus Pydantic models into the legacy-shaped structures the singletons build their cache entries from.

The adapter is a temporary shim that will be replaced by a standard legacy adapter once phase 3 ships the write API.

Scope

• All element types currently handled by the singletons: StorageElement (with its 4 access types — read, write, check, remove), ComputingElement, FTS, and Site.
• Mapping from diracx AllowedStatus / BannedStatus to the structures the singletons build their cache entries from.
• VO handling: iterate over all configured VOs and merge results.

Non-goals

• No wiring into the singletons (that's sub-issue C).
• No feature flag (that's sub-issue C).
• No caching inside the adapter — the singletons' existing RSSCache stays on top.

Reference

• Phase 1 Pydantic models: RSS data models (SQLAlchemy + Pydantic) #837
• Phase 2 main issue (this phase)

Acceptance criteria

• Module exposes a function per element type, accepting a diracx client and returning legacy-shaped data.
• Unit tests against a mocked diracx client cover every element type.
• Golden tests compare output to the format currently produced by the legacy RPC path.
• No dependency on DIRAC singletons, or the DIRAC CS.