mcp: allow Content-Type parameters in streamable transport (#878)

Use `mime.ParseMediaType` to parse the `Content-Type` header instead of
exact string matching. Previously, the server required an exact match on
`application/json`.

This allows the server to accept `application/json` with parameters
(e.g. `charset=utf-8`), even though [they are not
required](https://datatracker.ietf.org/doc/html/rfc4627.html#section-6),
following the robustness principle.

Per [RFC 8259 (section
8.1)](https://datatracker.ietf.org/doc/html/rfc8259.html#section-8.1),
`application/json` is always UTF-8, so such parameters are technically
redundant. However, parsing the media type makes the server more
tolerant of client behaviour, improving compatibility without
compromising security.

mcp/streamable_test.go

@@ -862,6 +862,14 @@ func TestStreamableServerTransport(t *testing.T) {
 					wantStatusCode: http.StatusOK,
 					wantMessages:   []jsonrpc.Message{resp(5, &CallToolResult{Content: []Content{}}, nil)},
 				},
+				{
+					// Correct Content-Type with parameters should pass.
+					method:         "POST",
+					headers:        http.Header{"Content-Type": {"application/json; charset=utf-8"}},
+					messages:       []jsonrpc.Message{req(5, "tools/call", &CallToolParams{Name: "tool"})},
+					wantStatusCode: http.StatusOK,
+					wantMessages:   []jsonrpc.Message{resp(5, &CallToolResult{Content: []Content{}}, nil)},
+				},
 			},
 			wantSessions: 1,
 		},