fix(gemini): coerce boolean JSON-schema sub-schemas for genai compatibility

[EronWright]

Problem

MCP tool input schemas are full JSON Schema, where any sub-schema position — a properties value, items, an anyOf entry, etc. — may be a bare boolean: true = "any value validates", false = "nothing validates". Gemini's Schema type is an OpenAPI-3.0 subset with no boolean-schema form, so genai unmarshals every sub-schema into a *genai.Schema and hard-fails on a JSON boolean:

json: cannot unmarshal bool into Go struct field Schema.properties of type genai.Schema

This kills the run before the first model call. A Go any/interface{} field in an MCP tool's input struct is the common source: schema generators such as google/jsonschema-go (used by mcp-go and the official MCP Go SDK) render an unconstrained field as the boolean true schema, so any MCP server exposing such a tool is otherwise unusable with Gemini.

Fix

Adds normalizeBooleanSchemas to the existing ConvertParametersToSchema normalization pipeline (alongside the type-array and enum normalizers already there): recursively coerce boolean sub-schemas to an empty object schema {} in every schema-bearing position (properties/items/anyOf/…), leaving additionalProperties booleans untouched (genai ignores the keyword). {} preserves "any value" while staying parseable by genai.

Tests

Adds pkg/model/provider/gemini/schema_boolean_test.go:

• TestConvertParametersToSchema_BooleanSubSchema — a schema with boolean sub-schemas (the any-field shape) now converts without error.
• TestNormalizeBooleanSchemas — booleans coerced in nested positions; additionalProperties booleans preserved.