Add CI test that validates all FML manifests are parseable by FmlClient

[jaredlockhart]

Because

• When application-services added stricter FML validation (bug 2020683, gecko-pref and default are now mutually exclusive), the Fenix v149.0.0 manifests became unparseable by FmlClient
• We had no unit test that actually loads each versioned FML manifest with FmlClient — the existing "Check Local Feature Manifests" CI job only checks that manifests are up-to-date with what the manifest tool generates, not that they pass FML validation
• This gap meant the breakage was only caught by integration tests timing out on a 500, not by a clear unit test failure
• See PR chore(nimbus): Update application-services #15002 and issue FmlClient constructor crash causes 500 when manifest has validation errors #15018 for the full investigation

This issue

• Add a unit test in experimenter/features/tests/test_features.py (alongside TestValidateFeatureManifests) that iterates all Application × Channel × Version combinations and calls FmlClient(path, channel) for each manifest file
• Use self.subTest() so failures identify the exact app/channel/version
• This ensures that application-services updates and manifest syncs are caught early with a clear error message

Blocked by

• Bug 2025587 — the Fenix v149.0.0 manifest needs to be fixed upstream first (remove default from gecko-nimbus-validation.test-preference), otherwise this test will fail immediately on the current manifests

┆Issue is synchronized with this Jira Task