fix: allow custom provider models in generated config schema

[BetterAndBetterII]

Issue for this PR

Closes #46248

Type of change

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

What does this PR do?

restoreModelRefs in packages/opencode/script/schema.ts was applying $ref: https://models.dev/model-schema.json#/$defs/Model as a sibling on every model / small_model string field. That $defs/Model is a closed enum, so a valid custom provider id such as internal/Qwen/Qwen3-Coder-30B-A3B-Instruct failed generated-schema validation even though runtime ConfigV1.model is a plain string.

This change emits anyOf: [{ type: "string" }, { $ref }] instead of the sibling $ref, so custom ids validate and the models.dev branch can still be used for autocomplete. The same helper covers top-level model / small_model and nested agent/command model fields.

How did you verify your code works?

cd packages/opencode && bun test test/script/schema.test.ts

3 tests: generated Config.model / small_model keep a string branch (no closed-enum-only $ref); the issue's custom provider model validates; a normal models.dev id still validates.

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Checklist

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

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