Add Trace-level logging for JSON-RPC payloads in transports

[Copilot]

This PR enables logging of all JSON-RPC payloads at the Trace level for visibility into wire-level protocol traffic.

Summary

Changes Made

Logger Methods in TransportBase:

• LogTransportSendingMessageSensitive(string, string) - (Trace level) logs the full JSON payload
• LogTransportSendingMessageSensitive(JsonRpcMessage) - overload that encapsulates the IsEnabled check and serialization (only this overload needs internal guarding since it does serialization)

Updated Transports (simplified - no external guards needed since arguments are cheap):

• StreamServerTransport - server-side stream transport
• StreamClientSessionTransport - client-side stream transport
• SseClientSessionTransport - client-side SSE transport
• StreamableHttpClientSessionTransport - client-side HTTP transport

Tests Added:

• SendMessageAsync_Should_Log_At_Trace_Level
• ReadMessagesAsync_Should_Log_Received_At_Trace_Level

Usage

To enable JSON-RPC payload logging, configure the logging level to Trace:

builder.Logging.SetMinimumLevel(LogLevel.Trace);

When Trace logging is enabled, you'll see messages like:

trce: Server (stream) transport sending message. Message: '{"jsonrpc":"2.0","method":"tools/list","id":1}'
trce: Server (stream) transport received message. Message: '{"jsonrpc":"2.0","result":{"tools":[]},"id":1}'

The LogTransportSendingMessageSensitive(JsonRpcMessage) overload internally checks IsEnabled(LogLevel.Trace) before serializing to avoid expensive serialization when trace logging is disabled. For call sites passing already-serialized strings, no external guard is needed since the source-generated logger methods handle the check internally.

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[stephentoub]

@copilot it looks like you've only adding tracing for sent payloads... what about received payloads?

[Copilot]

@copilot it looks like you've only adding tracing for sent payloads... what about received payloads?

The received message logging already existed via LogTransportReceivedMessageSensitive. I've now added IsEnabled(LogLevel.Trace) guards around those calls as well in 669781d to ensure consistency with the sent message logging.

[halter73]

Please use LoggedTest.LoggerFactory and LoggedTest.MockLoggerProvider in the newly added tests.

[halter73]

We could consider adding similar logging to SseResponseStreamTransport and StreamableHttpServerTransport. It would require adding an optional ILoggerFactory constructor parameter which we probably want to do anyway to avoid swallowing ISseEventStreamWriter errors (#1208), and it might be too noisy on a typical HTTP server to be useful without an explicit opt-in (which we could maybe base on IsDevelopment() in ASP.NET Core apps) beyond trace-level logging. That would definitely be a separate PR though.

To get a similarly verbose logging in Kestrel that includes request and response payloads, you have to explicitly add connection logging middleware. I was going to suggest maybe adding message filters (#1207) to the client in addition to the server to cover this scenario, but given that clients will typically have a specific ILoggerFactory explicitly passed in as a parameter rather than provided ambiently by DI, I think it's fine for trace level logging to be this verbose. It appears it already was for incoming messages.

[halter73]

@copilot Can you use the LoggerFactory property from the base class and get rid of the local mockLoggerProvider variable? By using the LoggerFactory property from LoggedTest, the MockLoggerProvider should be wired up automatically, so you shouldn't need the traceLoggerFactory either. Please update both of the new tests.

[Copilot]

@copilot Can you use the LoggerFactory property from the base class and get rid of the local mockLoggerProvider variable? By using the LoggerFactory property from LoggedTest, the `MockLoggerPr...

Updated the tests in 843ccca to use LoggerFactory and MockLoggerProvider from the base class. The LoggerFactory is overridden in the constructor to set LogLevel.Trace as the minimum level.