fix: add Accept header for MCP Streamable HTTP protocol compliance

The gateway returns 406 Not Acceptable when the Accept header doesn't
include both application/json and text/event-stream, as required by
the MCP Streamable HTTP transport spec (2025-03-26).

Add 'Accept: application/json, text/event-stream' to all fetch calls
in TC-SH-014 and TC-SH-015 E2E tests.

Signed-off-by: Myko <myko@mcpmux.com>

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Mohammod Al Amin Ashik <maa.ashik00@gmail.com>

Author: its-mash

tests/e2e/specs/streamable-http.wdio.ts

@@ -348,10 +348,12 @@ describe('Streamable HTTP: OAuth MCP Client Flow', function () {
     console.log('[test] Token obtained, length:', token.length);
 
     // Send MCP initialize request
+    // Streamable HTTP requires Accept: application/json, text/event-stream
     const res = await fetch(`http://localhost:${gatewayPort}/mcp`, {
       method: 'POST',
       headers: {
         'Content-Type': 'application/json',
+        'Accept': 'application/json, text/event-stream',
         'Authorization': `Bearer ${token}`,
       },
       body: JSON.stringify({
@@ -429,6 +431,7 @@ describe('Streamable HTTP: OAuth MCP Client Flow', function () {
       method: 'POST',
       headers: {
         'Content-Type': 'application/json',
+        'Accept': 'application/json, text/event-stream',
         'Authorization': `Bearer ${token}`,
       },
       body: JSON.stringify({
@@ -461,6 +464,7 @@ describe('Streamable HTTP: OAuth MCP Client Flow', function () {
       method: 'POST',
       headers: {
         'Content-Type': 'application/json',
+        'Accept': 'application/json, text/event-stream',
         'Authorization': `Bearer ${token}`,
         'Mcp-Session-Id': sessionId!,
       },
@@ -479,6 +483,7 @@ describe('Streamable HTTP: OAuth MCP Client Flow', function () {
       method: 'POST',
       headers: {
         'Content-Type': 'application/json',
+        'Accept': 'application/json, text/event-stream',
         'Authorization': `Bearer ${token}`,
         'Mcp-Session-Id': sessionId!,
       },