mcp: Implement server-side support for discover method (SEP-2575) (#987)

## Summary
Part 3 of the SEP-2575 ("Make MCP Stateless") implementation. Builds on
PR #975 which landed the client-side discover/fallback flow and the
per-request `_meta` plumbing. This PR adds the **server-side**
counterparts:

- `server/discover` handler (replaces the
stub),ProtocolVersionSupporter`
- transport hook for per-transport version filtering,
- transport-agnostic version validation and method-removal enforcement
in `ServerSession.handle`,
- HTTP-level wire compliance for SEP-2575 error codes/status codes,
- bug fix in `internal/jsonrpc2`,`processResult` with an `MCPGODEBUG`
opt-out for backward compatibility.

docs/mcpgodebug.md

@@ -39,6 +39,17 @@ Options listed below were added and will be removed in the 1.9.0 version of the
   restoring the previous behavior. The default behavior was changed so that
   stateless servers ignore session IDs entirely and reject `DELETE` with 405.
 
+- `nomethodnotfoundcodeinerror` added. If set to `1`, the jsonrpc2 layer will not
+  include the MethodNotFound Error (`-32601`) in the error response when the 
+  requested method in STDIO transport is not found. The default behavior was
+  changed to include the MethodNotFound Error in the error response when the
+  requested method in STDIO transport is not found.
+
+- `noprotocolerrorbody` added. If set to `1`, the streamable HTTP client will
+  not attempt to decode the JSON-RPC error body of a non-2xx HTTP response, 
+  restoring the previous behavior. The default behavior was changed so that
+  the client always attempts to surface the underlying JSON-RPC error.
+
 ### 1.6.0
 
 Options listed below were added and will be removed in the 1.8.0 version of the SDK.