feat: implement custom provider management with CRUD support, registration schema updates, and a dedicated UI form.

[ankitranjan7]

Summary

Implemented admin dashboard support for custom providers.

• Added create, edit, and delete flows for custom providers from the Providers dashboard.
• Added structured provider forms with dropdowns for finite choices like provider type, auth type, flow, OAuth auth method, device token request mode, and API-key header prefix.
• Added client-side validation with field-level highlighting for missing/invalid fields, while preserving the save-level error message.
• Added server-side validation for URL-like provider fields and API targets.
• Added PUT /api/providers/{provider} for updating custom providers.
• Fixed browser-session dashboard calls so custom provider mutations do not require a PoP header.
• Kept bundled providers read-only in the UI.
• Added focused regression tests for provider update, URL validation, and browser-session provider creation.