FmlClient constructor crash causes 500 when manifest has validation errors

[jaredlockhart]

Because

• The FmlClient constructor (from nimbus_megazord.fml) can raise FmlError.ValidationError when a feature manifest has validation errors (e.g., a variable with both gecko-pref and default, which are now mutually exclusive as of application-services build 150.20260321050323)
• NimbusFmlLoader.fml_client() does not catch this exception, so it propagates up and causes a 500 error on any page that calls get_invalid_fields_errors() (including the audience page during experiment creation)
• This was discovered via failing Fenix integration tests on PR chore(nimbus): Update application-services #15002 (application-services version bump) — all FIREFOX_FENIX tests failed with TimeoutException because the audience page returned a 500

This issue

• Adds a try/except FmlError around the FmlClient() constructor call in NimbusFmlLoader.fml_client() so that invalid manifests are logged and skipped rather than crashing the page
• Adds unit tests verifying the graceful degradation

Related:

• Bugzilla bug 2025587 — upstream Fenix manifest fix for the gecko-nimbus-validation.test-preference variable
• PR chore(nimbus): Update application-services #15002 — application-services bump that exposes this

┆Issue is synchronized with this Jira Task