feat(oauth2): opt-in claim-dump diagnostics for OIDC login failures (#6456)

# Description of Changes

## What & why

Customers using ADFS (or any generic OIDC provider that doesn't emit
`email`) hit `Attribute value for 'email' cannot be null` during OAuth2
login with no visibility into what claims the provider actually sent.
The only available remedy was guessing at
`security.oauth2.useAsUsername` until something worked.

This PR adds a new opt-in `security.oauth2.debugLogging` flag (default
`false`). When enabled, `CustomOAuth2UserService` logs:

- All ID token claims (sorted, with values)
- All UserInfo endpoint claims (if any)
- The merged attribute key set Spring exposes to `getAttribute()`
- The value the configured `useAsUsername` actually resolved to
- A **`Hint:`** line listing the claim keys present in the token that
map to a valid `UsernameAttribute` enum value — i.e. exactly what the
operator could put in `useAsUsername` to make login work

Logged at `INFO` on the success path and `ERROR` on failure (inside the
existing `catch (IllegalArgumentException)` block that throws
`OAuth2AuthenticationException`). The block is wrapped with a `[OAUTH2
DEBUG] ... [/OAUTH2 DEBUG]` banner and ends with a PII warning so
operators don't leave it on in production.

Default off → zero observable change for anyone not actively
troubleshooting.

## Files changed

| File | Why |
|---|---|
| `app/common/.../ApplicationProperties.java` | New `debugLogging` field
on the `OAUTH2` config class with javadoc warning about PII |
| `app/core/src/main/resources/settings.yml.template` | Documents
`oauth2.debugLogging` so it appears on next startup |
| `app/proprietary/.../security/service/CustomOAuth2UserService.java` |
Emits the claim dump + suggestion hint when the flag is on |
|
`app/proprietary/.../security/service/CustomOAuth2UserServiceDebugLoggingTest.java`
(new) | Unit test: mocks the OIDC delegate, asserts off-path is silent
and on-path emits the dump with the right Hint contents |

## End-to-end verification

Ran the bundled `testing/compose/docker-compose-keycloak-oauth.yml`
Keycloak realm, configured `security.oauth2.useAsUsername: mail`
(Keycloak emits `email`, not `mail`) and `provider: demarest` (matches
the original customer bug report). Triggered the OAuth flow at
`http://localhost:8080/oauth2/authorization/demarest` and confirmed:

- The ERROR-level dump fires with the full 19-claim ID token decoded
- `-- Value at 'mail' : <NULL — this is why login fails>` correctly
identifies the missing claim
- `-- Hint:` correctly suggests `[email, family_name, given_name,
preferred_username]` (the four keys present that map to valid
`UsernameAttribute` values)
- Auth still fails with the original `OAuth2AuthenticationException` —
no change to control flow, just added diagnostic logging

Unit test (`CustomOAuth2UserServiceDebugLoggingTest`) covers both
branches.

## Reviewer notes

- **No new public APIs.** The flag is config-only; no servlet endpoints
exposed.
- **PII is logged when the flag is on.** This is the whole point —
operators need to see the claims to fix their config — but it's gated,
defaults off, and the dump self-documents with a `WARNING: ... Set
security.oauth2.debugLogging=false once troubleshooting is complete.`
footer.
- **Why log everything, not just sub/email?** Because the operator
doesn't know in advance which claim they actually want. ADFS uses `upn`
in some configs and `preferred_username` in others; Azure AD uses `oid`;
the customer here had neither. Dumping the full set is the only way to
make the diagnostic self-service.
- **Out of scope for this PR (follow-ups):**
- The `UsernameAttribute` enum doesn't include `upn` / `unique_name`
(common ADFS claims). If the customer's token only has `upn`, the Hint
will be empty even though the operator can see `upn` in the dump. Worth
a separate PR to extend the enum.
- The known-provider validator in `Provider.java` (rejects e.g.
`useAsUsername: mail` for `provider: keycloak` at startup) bypasses our
diagnostic for those provider names. ADFS customers using `provider:
<name>` fall into the `default` branch so are not affected — but it's a
sharp edge worth documenting.

---

## Checklist

### General

- [x] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [x] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable) — N/A, backend-only change
- [x] I have performed a self-review of my own code
- [x] My changes generate no new warnings

### Documentation

- [ ] Doc-repo update (if functionality has heavily changed) —
diagnostic flag is self-documenting via the `settings.yml.template`
comment and the in-log warning; happy to add a doc-repo entry if
reviewers want one
- [ ] Translation tags — N/A

### UI Changes (if applicable)

- [ ] N/A — backend-only

### Testing (if applicable)

- [x] Unit test added (`CustomOAuth2UserServiceDebugLoggingTest`)
covering on/off paths and Hint correctness
- [x] End-to-end verified locally against bundled Keycloak compose with
intentionally misconfigured `useAsUsername`
- [x] Full `:proprietary:test` suite passes

app/core/src/main/resources/settings.yml.template

@@ -20,6 +20,7 @@ security:
     password: "" # initial password for the first login
   oauth2:
     enabled: false # set to 'true' to enable login (Note: enableLogin must also be 'true' for this to work)
+    debugLogging: false # set to 'true' to log full ID token and UserInfo claims during OAuth2/OIDC login. Use this to diagnose claim issues (e.g. "Attribute value for 'email' cannot be null" with ADFS). WARNING: writes PII (sub, email, name) to logs; disable after troubleshooting.
     client:
       keycloak:
         issuer: "" # URL of the Keycloak realm's OpenID Connect Discovery endpoint