Add GitHub MCP Server via Github app

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### Dify version

1.14.2

### Cloud or Self Hosted

Self Hosted (Docker)

### Steps to reproduce

1. Add an MCP server pointing to `https://api.githubcopilot.com/mcp/`
2. Add Github OAuth app 's client id and secret
2. Trigger the OAuth authorization flow (no static Authorization header).
3. Authorization fails

### ✔️ Expected Behavior

MCP server could be authenticated with a GH app, not just by adding a PAT via header (as #22808)

### ❌ Actual Behavior

```
ERROR [auth_client.py:122] Authentication retry failed
Traceback (most recent call last):
raise MCPAuthError(response=response_or_error.response)
core.mcp.error.MCPAuthError: Authentication failed

During handling of the above exception, another exception occurred:
Traceback (most recent call last):
raise ValueError(
ValueError: Client credentials token request failed: HTTP 404, Response: Not Found

During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "/app/api/core/mcp/auth_client.py", line 94, in _handle_auth_error
raise ValueError(f"Client credentials flow failed: {e}")
ValueError: Client credentials flow failed: Client credentials token request failed: HTTP 404, Response: Not Found
```