refactor(provider): split transform by family

[Mayhem50]

Issue for this PR

Follow-up refactor around #21237

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

This refactors ProviderTransform from a single transform.ts file into a structured transform/ module with a dispatching index.ts, shared helpers, and family-specific implementations under transform/family/.

The refactor keeps the public ProviderTransform API intact while making the provider logic easier to follow and evolve:

• provider families are resolved centrally in kind.ts
• normalize and cache are dispatched through the family implementation
• provider/model-specific variants logic now lives in family files instead of the orchestrator
• the previous catch-all misc bucket was split into explicit families (generic, groq, xai, venice, opencode, sap, mistral, custom)
• the existing Azure gpt-5 guard for reasoningSummary / textVerbosity is preserved in the refactored structure

How did you verify your code works?

• Ran bun test test/provider/transform.test.ts test/session/llm.test.ts test/session/prompt.test.ts test/provider/provider.test.ts
• Ran bun typecheck

Screenshots / recordings

Not applicable.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR