feat: expose token_endpoint_auth_method in gateway admin UI

[ecthelion77]

✨ Feature PR

📌 Summary

Expose the OAuth2 token_endpoint_auth_method parameter in the gateway add/edit forms, allowing administrators to select between client_secret_post (default) and client_secret_basic.

Some OAuth2 providers (e.g., Freshworks/Freshservice) require client_secret_basic (HTTP Basic Auth header) instead of the default client_secret_post (credentials in POST body). Without UI support, reconfiguring a gateway through the admin panel silently drops the token_endpoint_auth_method setting, causing token exchange failures.

Fixes #3991

🧭 Type of Feature

• Enhancement to existing functionality

💡 Implementation

admin.py: Read oauth_token_endpoint_auth_method from form data in both admin_add_gateway() and admin_edit_gateway(), and include it in oauth_config when present.

admin.html: Add a <select> dropdown in both the "Add Gateway" and "Edit Gateway" OAuth form sections, with options:

• client_secret_post — credentials in POST body (default)
• client_secret_basic — HTTP Basic Auth header

admin.js: In the editGateway() function, retrieve and populate the dropdown with the existing token_endpoint_auth_method value from the gateway's OAuth config.

The backend already supports token_endpoint_auth_method in oauth_config — this change only adds the UI surface.

🧪 Verification

Check	Command	Status
Lint suite	make lint	⚠️ Not run (no local dev env)
Unit tests	make test	⚠️ Not run (no local dev env)
Coverage ≥ 80 %	make coverage	⚠️ Not run (no local dev env)
Manual verification	Tested in production (Kubernetes) for 3+ weeks — Freshservice gateway configured via UI with client_secret_basic, token exchange succeeds. Editing the gateway preserves the setting.	✅

📐 MCP Compliance

• Matches current MCP spec
• No breaking change to MCP clients
• token_endpoint_auth_method is defined in RFC 6749 Section 2.3 and RFC 7591 Section 2

✅ Checklist

• Code formatted
• No secrets/credentials committed
• DCO Signed-off-by included
• UI changes follow existing Tailwind CSS patterns

[ecthelion77]

Suggested labels: enhancement, COULD, python, javascript, ui