fix: deprecate USE_FIXED_HTTP for SSE streaming support (Issue #524)

The fixed HTTP implementation does not support SSE streaming required
by clients like OpenAI Codex. This commit deprecates USE_FIXED_HTTP
and makes SingleSessionHTTPServer the default.

Changes:
- Add deprecation warnings in src/mcp/index.ts and src/http-server.ts
- Remove USE_FIXED_HTTP from docker-compose.yml and Dockerfile.railway
- Update .env.example with deprecation notice
- Rename npm script to start:http:fixed:deprecated
- Update all documentation to remove USE_FIXED_HTTP references
- Mark test case as deprecated

Users should unset USE_FIXED_HTTP to use the modern SingleSessionHTTPServer
which supports both JSON-RPC and SSE streaming.

Closes #524

Concieved by Romuald Członkowski - www.aiadvisors.pl/en

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

scripts/test-url-configuration.ts

@@ -71,10 +71,12 @@ const testCases: TestCase[] = [
     }
   },
   {
-    name: 'Fixed HTTP implementation',
+    // DEPRECATED: This test case tests the deprecated fixed HTTP implementation
+    // See: https://github.com/czlonkowski/n8n-mcp/issues/524
+    name: 'Fixed HTTP implementation (DEPRECATED)',
     env: {
       MCP_MODE: 'http',
-      USE_FIXED_HTTP: 'true',
+      USE_FIXED_HTTP: 'true',  // DEPRECATED: Will be removed in future version
       AUTH_TOKEN: 'test-token-for-testing-only',
       PORT: '3005',
       BASE_URL: 'https://fixed.example.com'