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