fix: sanitize tool schemas for strict JSON Schema validators (#13737)

[echoVic]

Problem

Built-in tool schemas (like question) break on providers with strict JSON Schema validation — Codex, Vertex AI OpenAI-compatible, SGLang, etc.

Two issues:

1. Zod .meta() pollution: Injects $schema and ref keywords that are invalid in OpenAI function parameter schemas
2. Missing required fields: When additionalProperties: false is set, strict validators require ALL property keys to be in the required array. Optional fields like multiple are missing.

Error from Codex:

Invalid schema for function 'question': In context=('properties', 'questions',
'items'), 'required' is required to be supplied and to be an array including
every key in properties. Missing 'multiple'.

Fix

Add universal schema sanitization in ProviderTransform.schema() that runs before any provider-specific transforms:

1. Strip Zod meta fields ($schema, ref) recursively from all schemas
2. Fix strict mode compliance: When additionalProperties: false is set, ensure all property keys are in required. Optional properties are wrapped with anyOf: [original, {type: 'null'}] so they remain nullable while satisfying strict validators.

Changes

• packages/opencode/src/provider/transform.ts: Add sanitizeStrict() pass in schema() function

Closes #13737
Related: #13618, #11413, #8184

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

I found a potential related PR:

PR #13738: fix(provider): sanitize tool schemas for strict validators
#13738

This appears to be addressing the same issue - sanitizing tool schemas for strict JSON Schema validators. Since PR #13823 is the current PR and PR #13738 has a similar title and purpose, these are likely duplicate efforts on the same problem.

You may want to check if one of these PRs should be closed or if they're being developed in parallel with different approaches.