fix(flipt): Fix FliptProvider to honor server default variant when match is false

[Openroads]

This PR

Fixes Flipt variant evaluation fallback behavior so that the provider honors Flipt’s server-side default variant even when match=false. Previously, FliptProvider#evaluateVariant returned the OpenFeature SDK defaultValue whenever response.isMatch() was false, which caused the server-provided default variant (variantKey) to be ignored.

• uses the SDK defaultValue only when match=false and variantKey is null/empty
• preserves the server-returned variant (and value derived from the current variant logic) when variantKey is present, even if match=false
  adds a unit test covering the match=false + variantKey (server default) scenario

Notes

The new test uses a mocked /evaluate/v1/variant response with match=false and a non-empty variantKey to validate the behavior.

How to test

Run and verify the new test passes:
getStringVariantEvaluation_usesServerDefaultVariantWhenNoMatch

[gemini-code-assist]

Code Review

This pull request updates the Flipt provider to support server-provided default variants. Previously, if a targeting match was not found, the provider would immediately return the SDK's default value. The logic now checks for a server-provided variant key even when a match is false, allowing the server's default configuration to take precedence. A review comment identifies that the evaluation reason should be updated to 'DEFAULT' in these cases to comply with the OpenFeature specification, as the current implementation would incorrectly report 'TARGETING_MATCH'.