[ENHANCEMENT] Introduce canonical active and retired provider identifiers

[WebMad]

Problem (one or two sentences)

Active and retired provider identifiers are currently defined inside provider-settings.ts, instead of a lightweight shared module. This makes the identifiers harder to reuse and keep consistent

Context (who is affected and when)

This affects maintainers and contributors when adding, retiring, or reusing provider identifiers across the codebase. The current definitions in provider-settings.ts are coupled to the broader provider settings module

Desired behavior (conceptual, not technical)

Active and retired provider identifiers should have one lightweight shared source, while existing provider strings and public APIs remain unchanged

Constraints / preferences (optional)

No response

Request checklist

• I've searched existing Issues and Discussions for duplicates
• This describes a specific problem with clear context and impact

Zoo Code Task Links (optional)

#944

Acceptance criteria (optional)

No response

Proposed approach (optional)

No response

Trade-offs / risks (optional)

No response

[WebMad]

Claiming - I’d like to work on this issue

[WebMad]

Implementation is ready in #952