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.

mcp/server_test.go

@@ -1100,6 +1100,10 @@ func TestServerCapabilitiesOverWire(t *testing.T) {
 // that opts into the new protocol via `_meta.protocolVersion` must be
 // rejected with `Method not found` (-32601).
 func TestServerSessionHandle_RejectsInitializeOnNewProtocol(t *testing.T) {
+	orig := supportedProtocolVersions
+	supportedProtocolVersions = append([]string{protocolVersion20260630}, slices.Clone(orig)...)
+	t.Cleanup(func() { supportedProtocolVersions = orig })
+
 	tests := []struct {
 		name       string
 		params     any
@@ -1213,6 +1217,10 @@ func TestServerSessionHandle_RejectsInitializeOnNewProtocol(t *testing.T) {
 // `ping`) all return Method not found when the request opts into the new
 // protocol via `_meta.protocolVersion`.
 func TestServerSessionHandle_RejectsRemovedMethodsOnNewProtocol(t *testing.T) {
+	orig := supportedProtocolVersions
+	supportedProtocolVersions = append([]string{protocolVersion20260630}, slices.Clone(orig)...)
+	t.Cleanup(func() { supportedProtocolVersions = orig })
+
 	newProtoMeta := map[string]any{
 		"_meta": map[string]any{
 			MetaKeyProtocolVersion:    protocolVersion20260630,