fix(gemini): strip JSON-Schema keywords the tool API rejects - #4761

Problem

Gemini 400s the whole tool batch on a JSON-Schema keyword it has no field for:

Invalid JSON payload received. Unknown name "const" at
'tools[0].function_declarations[21].parameters.properties[3].value.items
 .properties[6].value.properties[0].value': Cannot find field.

Gemini's function-declaration schema is an OpenAPI 3.0 subset. Zod 4's z.toJSONSchema (draft 2020-12, used by zodToJsonSchema in packages/runtime/src/zod-schema.ts) emits const for every z.literal() and $ref + $defs for every reused schema — so any tool defined in Zod hit this. One bad keyword anywhere in the tree aborts every tool in the request, not just the offending one.

sanitizeGeminiSchema already stripped a handful of keywords (additionalProperties, $ref, patternProperties, …) but not const, and stripping $ref while dropping $defs left an empty schema behind.

Changes

packages/runtime/src/providers/gemini-provider.ts:

• const → an accepted form. A string literal becomes a single-value enum; a number or boolean literal becomes its type plus a Must be X. description hint, since Gemini's enum takes strings only.
• $ref inlined against the root document before $defs is dropped. A cyclic or unresolvable ref degrades to { type: "object" } rather than {}.
• oneOf → anyOf (same meaning to the model, and anyOf is in Gemini's dialect); allOf folded into the parent, merging properties and unioning required, with parent keys winning.
• ["string","null"] → type: "string" + nullable: true — Gemini's type is one string, and the union is Zod's nullable shape.
• Remaining draft-2020-12 keywords dropped: not, if/then/else, prefixItems, contains, uniqueItems, multipleOf, examples, readOnly/writeOnly, deprecated, contentEncoding/contentMediaType, $comment, $defs.
• Recursion is now schema-aware. Keyword filtering no longer applies to property names, so a property called const (or additionalProperties) survives, and data under enum/default/example is copied verbatim instead of being walked as a schema.

Existing behavior kept: tool-name sanitization, and the items backfill for array schemas.

Tests

Five cases added to packages/runtime/tests/providers/gemini-provider.test.ts: const rewriting for string/number/boolean literals, a property literally named const, $ref inlining with $defs removal, a recursive $ref, and the oneOf/allOf/nullable-union mapping. npx vitest run tests/providers/gemini-provider*.test.ts — 107 passed. npm run lint --workspace=packages/runtime (tsc) clean.

Unrelated pre-existing failures in tests/providers (fal, kie, atlascloud, manifest-models, replicate, together, topaz) are present on main in this environment and untouched by this change.

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