MCP parity: request metadata and 165 interaction tests (#11)

* Add 165 server interaction tests for Python parity

Comprehensive test coverage matching Python's test_server_interactions.py:
- TestTools, TestToolParameters, TestOutputSchema, TestContentTypes
- TestResource, TestResourceTemplates, TestResourceAnnotations
- TestPrompts, TestPromptVariations, TestMeta
- TestCapabilities, TestProgressAndNotifications, TestRootsNotification
- TestCancelledNotification, TestLogging, TestImageContent
- TestEmbeddedResource, TestToolInputValidation, TestResourceSubscribe
- TestCompletionEdgeCases, TestBatchOperations, TestTransportEdgeCases
- Unicode, pagination, large data, error handling, and edge cases

Also fixes iterator dereference bug in client.hpp when accessing
structuredContent without registered outputSchema.

* Add request meta fields to Context with coverage

* Add optional request metadata to Context and tests

* Fix UTF-16 encoding in context_meta.cpp

* Fix typed struct access in InitializeResult tests

The InitializeResult type is a typed struct, not JSON - use proper
struct field access instead of JSON-style accessor methods.

Author: 0xeb

tests/server/context_meta.cpp

@@ -1,4 +1,4 @@
-#include "fastmcpp/prompts/manager.hpp"
+#include "fastmcpp/prompts/manager.hpp"
 #include "fastmcpp/resources/manager.hpp"
 #include "fastmcpp/server/context.hpp"
 

tests/server/interactions.cpp

@@ -4168,6 +4168,7 @@ void test_server_info()
     client::Client c(std::make_unique<client::LoopbackTransport>(srv));
 
     auto info = c.initialize();
+    assert(!info.serverInfo.name.empty());
     assert(info.serverInfo.name == "test_server");
     assert(info.serverInfo.version == "1.0.0");