Disable authentication using ServiceAccount

[git-hyagi]

ref: https://issues.redhat.com/browse/PULP-1064

Summary by Sourcery

Bug Fixes:

• Prevent continued access when deprecated c.rh.c ServiceAccount authentication is used by failing the request with an authentication error.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

This PR changes the deprecated ServiceAccount-based authentication path so that any attempt to authenticate via the remote_user header now fails with an AuthenticationFailed error instead of silently falling back to unauthenticated access.

Sequence diagram for deprecated ServiceAccount authentication behavior change

sequenceDiagram
  actor ApiClient
  participant PulpAPI
  participant AuthenticationBackend

  ApiClient->>PulpAPI: HTTP request with remote_user header
  PulpAPI->>AuthenticationBackend: authenticate(request)
  AuthenticationBackend->>AuthenticationBackend: Parse request JSON
  alt Invalid JSON
    AuthenticationBackend-->>PulpAPI: raise AuthenticationFailed Access denied. Invalid JSON.
    PulpAPI-->>ApiClient: 401 Unauthorized
  else Valid JSON and remote_user present
    AuthenticationBackend-->>PulpAPI: raise AuthenticationFailed Deprecated c.rh.c SA authentication method attempted by user
    PulpAPI-->>ApiClient: 401 Unauthorized
  else No remote_user
    AuthenticationBackend-->>PulpAPI: return None (no authentication)
    PulpAPI-->>ApiClient: Process as unauthenticated request
  end

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File-Level Changes

Change	Details	Files
Reject deprecated ServiceAccount authentication attempts instead of ignoring them.	Update authentication flow to raise AuthenticationFailed when remote_user is present for the deprecated c.rh.c ServiceAccount method. Change warning-only behavior into a hard authentication failure to prevent access using the deprecated mechanism.	pulp_service/pulp_service/app/authentication.py

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[sourcery-ai]

Hey - I've found 1 issue, and left some high level feedback:

• Consider preserving the warning log when rejecting deprecated SA authentication (log then raise) so that operational visibility of these attempts is not lost.
• Including the remote_user value in the authentication failure message may leak user identifiers to clients; consider logging it server-side only and returning a more generic error to the caller.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Consider preserving the warning log when rejecting deprecated SA authentication (log then raise) so that operational visibility of these attempts is not lost.
- Including the `remote_user` value in the authentication failure message may leak user identifiers to clients; consider logging it server-side only and returning a more generic error to the caller.

## Individual Comments

### Comment 1
<location> `pulp_service/pulp_service/app/authentication.py:99-102` </location>
<code_context>
-            _logger.warning(f"Deprecated c.rh.c SA authentication method attempted by user: {remote_user}")
-            return None
+            raise AuthenticationFailed(
+                _(f"Deprecated c.rh.c SA authentication method attempted by user: {remote_user}")
+            )

</code_context>

<issue_to_address>
**suggestion:** Consider avoiding f-strings inside the translation function.

Using an f-string inside `_()` ties the translation key to the specific value, which makes catalog management harder. Prefer placeholders instead, e.g. `_("Deprecated c.rh.c SA authentication method attempted by user: %(user)s") % {"user": remote_user}`.

```suggestion
        if remote_user:
            raise AuthenticationFailed(
                _(
                    "Deprecated c.rh.c SA authentication method "
                    "attempted by user: %(user)s"
                )
                % {"user": remote_user}
            )
```
</issue_to_address>

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