fix(provider): preserve @cf model identifiers

[Alix-007]

Fixes #1165.

• treat Cloudflare @cf/... IDs as model paths, not protocol prefixes
• preserve the full model string in provider parsing and model refs
• add regression tests for CreateProviderFromConfig and ParseModelRef

[Alix-007]

Follow-up update: I pushed a forward-only sync commit so the Cloudflare model-ID fix now sits on top of the current provider matrix (azure, anthropic_messages, common, and protocoltypes). Local validation on this branch: go test ./pkg/providers -run TestExtractProtocol -count=1, go test ./pkg/providers -run TestCreateProviderFromConfig_CloudflareModelID -count=1, go test ./pkg/providers -run TestCreateProviderFromConfig_ModelScope -count=1, go test ./pkg/providers -run TestCreateProviderFromConfig_Azure -count=1, go test ./pkg/providers -run TestGetDefaultAPIBase_ModelScope -count=1, plus go test for ./pkg/providers/anthropic_messages, ./pkg/providers/azure, and ./pkg/providers/common.

[Alix-007]

Pushed a follow-up that moves the Cloudflare coverage away from the same insertion point as the new Azure cases. Local checks passed with:

• go test ./pkg/providers -run TestExtractProtocol -count=1
• go test ./pkg/providers -run TestCreateProviderFromConfig_CloudflareModelID -count=1
  I also re-ran a local merge probe against upstream/main and it now merges cleanly.

[Alix-007]

Superseded by #1701, which rebases the same @cf model-id preservation fix onto current main and now has a green PR workflow.