fix(go/plugins/googlegenai): fixed nullable in array type in JSON schema

[apascal07]

Fixes #5070.

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)

[gemini-code-assist]

Code Review

This pull request enhances the Google GenAI plugin's schema conversion logic to support JSON Schema 2020-12 features, specifically handling "type" arrays (e.g., ["string", "null"]) and improving "anyOf" subschema resolution. It also introduces a comprehensive test suite for these conversion scenarios. The review feedback identifies opportunities to make the "anyOf" logic more robust by recursively validating subschemas and to extend "type" field support to include []string for better compatibility with Go-defined schemas.

[gemini-code-assist]

The current logic for finding a valid sub-schema in anyOf only checks if the type field is a string. This will skip sub-schemas that use the newly supported array type format (e.g., {"type": ["string", "null"]}). A more robust approach is to recursively call toGeminiSchema and check if the resulting schema has a concrete type. This also simplifies the logic by leveraging the existing type parsing and normalization.

for _, subSchema := range anyOfList {
			if v, ok := genkitSchema["title"]; ok {
				subSchema["title"] = v
			}
			if v, ok := genkitSchema["description"]; ok {
				subSchema["description"] = v
			}
			res, err := toGeminiSchema(originalSchema, subSchema)
			if err == nil && res != nil && res.Type != 0 {
				return res, nil
			}
		}

[gemini-code-assist]

To be consistent with the anyOf handling and support schemas constructed directly in Go code, the type field should also accept []string in addition to []any.

case []any, []string:
		var list []any
		if l, ok := tv.([]any); ok {
			list = l
		} else {
			for _, s := range tv.([]string) {
				list = append(list, s)
			}
		}
		for _, t := range list {
			s, isString := t.(string)
			if !isString {
				return nil, fmt.Errorf("schema 'type' array contains non-string element of type %T", t)
			}
			if s == "null" {
				schema.Nullable = genai.Ptr(true)
				continue
			}
			if typeStr != "" {
				return nil, fmt.Errorf("schema 'type' array contains multiple non-null types: %v", tv)
			}
			typeStr = s
		}