fix: only show DCR hint for authorization_code grant type

[marekdano]

Closes IBM/mcp-context-forge#6466

Summary

"DCR hint shown on Client ID and Secret for the client credentials grant"

• The DCR (Dynamic Client Registration) hint under Client ID/Client Secret was shown for every OAuth grant type, but DCR only applies to the authorization-code flow (backend /oauth/authorize/{gateway_id}). Users on client_credentials who followed the hint and left those fields blank hit an opaque "OAuth authentication failed" error from register_gateway.
• Gate both dcrHelp hints on grantType === "authorization_code".
• Reword the hint (en-US/es-ES/pt-BR) from "Not required for servers that support Dynamic Client Registration (DCR)" to "Optional. Some authorization servers issue these automatically when you authorize." to avoid implying the fields can be skipped.

Test plan

• Added unit tests in OAuth2Auth.test.tsx covering hint visibility per grant type
• npx vitest run src/components/mcp-servers/OAuth2Auth.test.tsx — 18 passed
• npx tsc --noEmit clean