fix(flags): fix failed flag overwrite on individual evaluation error

[matheus-vb]

Problem

When the /flags endpoint fails to evaluate a specific flag due to a transient error (e.g. DB timeout), the server returns that flag with enabled: false and a new failed: true field. The SDK then includes this failed result when transforming the v4 response, which can overwrite a previously valid value. In getAllFlags(), array_merge overwrites a locally evaluated true with the server's erroneous false.

See PostHog/posthog#46204.

Changes

lib/Client.php

• In normalizeFeatureFlags(): skip flags with failed: true during the v4-to-v3 response transformation. Failed flags are excluded from featureFlags and featureFlagPayloads, so they cannot overwrite cached or locally evaluated values.
• In getFeatureFlagResult(): exclude failed flags from flagDetail lookup. This prevents a null access warning when reading $flagDetail['reason']['description'] for flags whose reason may be null, and avoids attaching invalid metadata to analytics events.

test/FeatureFlagErrorTest.php

• testFailedFlagIsFilteredOutByFailedField: verifies that the failed field drives the filtering. Uses three flags — failed=true, enabled=false+failed=false (legitimately disabled), and enabled=true+failed=false — to prove that only failed=true is filtered, not all disabled flags.
• testGetAllFlagsDoesNotOverwriteLocalResultWithFailedServerFlag: reproduces the actual overwrite scenario. A flag evaluates locally as true, the server returns it with failed=true, and the test asserts the local value is preserved after array_merge.

Tests

All 155 tests pass, no new PHP warnings introduced.

[dustinbyrne]

Looks great!