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

[vishu-bh]

LGTM 🚀

[a-effort]

Reviewed the diff. No blockers.

The DCR hint is now gated on grantType === 'authorization_code' in both the Client ID and Client Secret fields. The hint text is also reworded to remove the DCR acronym and describe the actual behavior. The i18n strings are updated across all three locales consistently.

The test coverage is correct: it checks that the hint is absent for client_credentials and password grant, and present twice (once per field) for authorization_code. The existing test for the password grant legacy label is preserved and unaffected.