fix(dippin-lang): knownModelProviders linter catalog is stale

[harperreed]

Summary

The model catalog in dippin-lang/validator/lint.go is hardcoded and missing:

• gemini as a provider (used extensively in examples)
• Models like gpt-5.2, claude-sonnet-4-5, claude-sonnet-4-6

Running dippin lint on most example files produces spurious DIP108 warnings.

Recommendation

Either update the catalog or make it configurable/extensible.

Sources

Found by: DSL Design Expert (expert panel review)

[clintecker]

Fixed in dippin-lang v0.17.0: new --extra-models flag on lint/doctor for custom model catalogs. No tracker-side change needed.