[Azure Search] Fix Offerings API shape to comply with ARM RPC (pre-GA)

Tracks ARM feedback on Offerings API from 2026-03-01-preview: https://github.com/Azure/azure-rest-api-specs/pull/40408#issuecomment-4057468054

Current API:
- `GET /providers/Microsoft.Search/offerings` returns a non-resource collection
- Under `/providers/...`, this is interpreted as a resourceType collection, which is not RPC-compliant
- Results in `RequiredPropertiesMissingInResourceModel`

Plan for PR https://github.com/Azure/azure-rest-api-specs/pull/40408
- Keep current shape for 2026-03-01-preview (avoid breaking preview consumers)
- Fix API shape in next release cycle (~6 months) and ensure compliance before GA

Fix Plan (committed with an internal tracking Feature item)
- Replace with action-style API, e.g. `POST /providers/Microsoft.Search/fetchOfferings`
- Remove `offerings` as a resourceType implication
- Evaluate removing from RP manifest if needed

Alignment
Confirmed with ARM in office hours:
- Current shape is incorrect
- Must be fixed before GA

Start with the ARM feedback linked from the issue and PR 40408, then inspect the 2026-03-01-preview Offerings API shape and the resource provider manifest. Done means the next release uses an action-style API without an offerings resourceType implication and passes ARM compliance checks before GA.

Written by the indexing model from the issue text.