feat(openai): add configurable Fable family mapping

[ncwn]

Summary

• classify Claude Fable requests as a first-class messages-dispatch family
• add an optional fable_mapped_model group setting before Opus, Sonnet, and Haiku
• keep exact-model mappings as higher-priority overrides
• leave Fable unmapped when the family setting is blank

Problem

Fable was missing from the shared Claude family classifier. A group-level exact mapping could resolve a target, but downstream OpenAI forwarding only applied the messages-dispatch target to a recognized Claude family. The original Fable model ID could therefore reach a Codex account and be rejected as unsupported.

A family setting is more reliable for versioned Fable model IDs while retaining exact mappings for genuine exceptions. The target model remains deployment configuration; this change does not add a built-in Fable target.

Tests

• make test-unit
• make test-integration
• make test-frontend
• go run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.9.0 run ./...

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[ncwn]

I have read the CLA Document and I hereby sign the CLA