Gemini rejects tool declarations with empty-string enum values (answer_commit) #1531

Every request to a Gemini model (google/* or google-vertex/*) that includes the analyst agent's answer_commit tool fails before the model runs:

APIError (status 400): * GenerateContentRequest.tools[0].function_declarations[44].parameters.properties[unresolved].any_of[1].enum[0]: cannot be empty

answer_commit wraps its optional fields in a union that accepts "" (strict-schema providers fill unused properties with empty strings). That z.literal("") is emitted as const: "" / enum: [""], and the Gemini API rejects empty-string enum values. The Gemini branch of ProviderTransform.schema stringifies enum values but does not drop empty ones.

Expected: the Gemini schema sanitizer removes empty-string enum/const values, so any tool whose schema contains them can be declared to Gemini. Runtime Zod validation of tool arguments is unchanged.