Fix case-sensitive Accept header comparison

## Motivation and Context

RFC 9110 defines media type comparison as case-insensitive,
but `parse_accept_header` compared types without normalizing case.
This caused `Application/JSON` or `TEXT/Event-Stream` to be rejected
with 406 Not Acceptable.

## How Has This Been Tested?

Added tests for mixed-case Accept headers:

- `Accept: Application/JSON, Text/Event-Stream` succeeds for POST
- `Accept: TEXT/EVENT-STREAM` succeeds for GET SSE

## Breaking Changes

None. This is a bug fix bringing behavior into conformance with RFC 9110.

test/mcp/server/transports/streamable_http_transport_test.rb

@@ -1465,6 +1465,45 @@ module MCP
           assert_equal 200, response[0]
         end
 
+        test "POST request with mixed-case Accept header succeeds" do
+          request = create_rack_request_without_accept(
+            "POST",
+            "/",
+            {
+              "CONTENT_TYPE" => "application/json",
+              "HTTP_ACCEPT" => "Application/JSON, Text/Event-Stream",
+            },
+            { jsonrpc: "2.0", method: "initialize", id: "123" }.to_json,
+          )
+
+          response = @transport.handle_request(request)
+          assert_equal 200, response[0]
+        end
+
+        test "GET request with upper-case Accept header succeeds" do
+          init_request = create_rack_request(
+            "POST",
+            "/",
+            { "CONTENT_TYPE" => "application/json" },
+            { jsonrpc: "2.0", method: "initialize", id: "123" }.to_json,
+          )
+          init_response = @transport.handle_request(init_request)
+          session_id = init_response[1]["Mcp-Session-Id"]
+
+          request = create_rack_request_without_accept(
+            "GET",
+            "/",
+            {
+              "HTTP_ACCEPT" => "TEXT/EVENT-STREAM",
+              "HTTP_MCP_SESSION_ID" => session_id,
+            },
+          )
+
+          response = @transport.handle_request(request)
+          assert_equal 200, response[0]
+          assert_equal "text/event-stream", response[1]["Content-Type"]
+        end
+
         test "GET request without Accept header returns 406" do
           init_request = create_rack_request(
             "POST",