fix: tool param not set to default value when omitted

[gregnr]

When defining a tool call, you can give a param a default value but it doesn't actually use it if the param was omitted. This fixes that.

const server = createMcpServer({
  name: 'test-server',
  version: '0.0.0',
  tools: {
    search: tool({
      description: 'Search text',
      parameters: z.object({
        query: z.string(),
        caseSensitive: z.boolean().default(false),
      }),
      execute: async (args) => {
        return args;
      },
    }),
  },
});

// Call the tool without the optional parameter
const result = await callTool({
  name: 'search',
  arguments: {
    query: 'hello',
  },
});

expect(result).toEqual({
  query: 'hello',
  caseSensitive: false,
});