Fix SDK types to align with MCP spec for sampling tools

Changes:
- Remove `disable_parallel_tool_use` from ToolChoiceSchema (not in MCP spec)
- Remove unnecessary `.passthrough()` from ToolChoiceSchema
- Change CreateMessageResultSchema.role from z.literal("assistant") to
  z.enum(["user", "assistant"]) to match spec's SamplingMessage.role
- Update spec type count from 123 to 127 (4 new sampling tool types)
- Fix test accessing .type on content union (could be array)
- Add test for CreateMessageResult with array content
- Remove test expecting user role to fail (spec allows both roles)

Note: 7 type compatibility errors remain due to upstream spec issue
where ToolUseContent.input and ToolResultContent.structuredContent use
`object` type instead of `{ [key: string]: unknown }`.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: bhosmer-ant

src/spec.types.test.ts

@@ -647,7 +647,7 @@ describe('Spec Types', () => {
     it('should define some expected types', () => {
         expect(specTypes).toContain('JSONRPCNotification');
         expect(specTypes).toContain('ElicitResult');
-        expect(specTypes).toHaveLength(123);
+        expect(specTypes).toHaveLength(127);
     });
 
     it('should have up to date list of missing sdk types', () => {