test(mcp): Simulate stdio transport with memory streams (#5329)

Stop mocking transport layer of the `mcp` package in tests.

Convert functions annotated with `@mcp.call_tool`, and analogous resource and prompt decorators, to async if, and only if, an exception is raised when going through the transport layer.

Remove `test_resource_without_protocol`, `test_request_context_no_context` and `test_async_handlers_mixed`, because they are testing unreachable states. The former two are only possible when bypassing the transport layer. The latter is impossible because an exception is raised when invoking the synchronous `@mcp.call_tool`-annotated function.

Author: alexander-alderman-webb