fix: remove propertyNames from JSON schema for Gemini compatibility

Gemini API does not support the JSON Schema `propertyNames` keyword,
causing 400 errors when Claude tool schemas containing this field are
proxied through the Antigravity provider.

Add `propertyNames` to the list of unsupported keywords removed by
CleanJSONSchemaForGemini(), alongside existing removals like $ref,
definitions, and additionalProperties.

internal/util/gemini_schema.go

@@ -296,6 +296,7 @@ func flattenTypeArrays(jsonStr string) string {
 func removeUnsupportedKeywords(jsonStr string) string {
 	keywords := append(unsupportedConstraints,
 		"$schema", "$defs", "definitions", "const", "$ref", "additionalProperties",
+		"propertyNames", // Gemini doesn't support property name validation
 	)
 	for _, key := range keywords {
 		for _, p := range findPaths(jsonStr, key) {