fix(schema): strip contentEncoding from MCP tool schemas for Gemini compatibility

The existing normalizeToolArgSchemas only applies to omo plugin tools
(via tool-registry.ts), but MCP server tool schemas bypass this
sanitization entirely. MCP schemas with contentEncoding/contentMediaType
cause Gemini 400 errors.

Add sanitizeJsonSchema() to strip unsupported keywords from MCP tool
inputSchema before serialization in formatMcpCapabilities.

Fixes #2200
Supersedes #2666

src/plugin/normalize-tool-arg-schemas.ts

@@ -40,3 +40,37 @@ export function normalizeToolArgSchemas<TDefinition extends Pick<ToolDefinition,
 
   return toolDefinition
 }
+
+// Schema keywords unsupported by Gemini — strip them from MCP tool schemas
+const UNSUPPORTED_SCHEMA_KEYWORDS = new Set(["contentEncoding", "contentMediaType"])
+
+function isRecord(value: unknown): value is Record<string, unknown> {
+  return typeof value === "object" && value !== null && !Array.isArray(value)
+}
+
+export function sanitizeJsonSchema(value: unknown, depth = 0, isPropertyName = false): unknown {
+  if (Array.isArray(value)) {
+    return value.map((item) => sanitizeJsonSchema(item, depth + 1, false))
+  }
+
+  if (!isRecord(value)) {
+    return value
+  }
+
+  const sanitized: Record<string, unknown> = {}
+
+  for (const [key, nestedValue] of Object.entries(value)) {
+    if (!isPropertyName && UNSUPPORTED_SCHEMA_KEYWORDS.has(key)) {
+      continue
+    }
+
+    if (depth === 0 && key === "$schema") {
+      continue
+    }
+
+    const childIsPropertyName = key === "properties" && !isPropertyName
+    sanitized[key] = sanitizeJsonSchema(nestedValue, depth + 1, childIsPropertyName)
+  }
+
+  return sanitized
+}