fix: guard against nil ContextEvaluator func in memprovider Resolve

[somaz94]

memprovider.InMemoryFlag.ContextEvaluator is typed *func(...). Resolve
only checked the pointer for nil, so a non-nil pointer to a nil func passed the
guard and panicked on the dereference-and-call. This adds the missing inner
nil-func check so such a flag falls back to the default variant, plus a
regression test.

Fixes #521

Validation:

• make test (go test -tags testtools -race): all packages pass, memprovider at 86.9% coverage
• make lint (golangci-lint v2.9.0): 0 issues
• Confirmed the new test panics against the old guard and passes with the fix

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: acfd78f5-08c1-4116-8099-9e74263118bd

📥 Commits

Reviewing files that changed from the base of the PR and between e75753b and d7ee7b6.

📒 Files selected for processing (2)

• openfeature/memprovider/in_memory_provider.go
• openfeature/memprovider/in_memory_provider_test.go

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📝 Walkthrough

Walkthrough

Modified InMemoryFlag.Resolve in the memprovider package to check both the ContextEvaluator pointer and the underlying function value for nil before invoking it, preventing a nil-pointer dereference panic. Added a regression test verifying fallback to DefaultVariant when ContextEvaluator points to a nil function.

Changes

ContextEvaluator nil guard

Layer / File(s)	Summary
Nil-function guard and regression test openfeature/memprovider/in_memory_provider.go, openfeature/memprovider/in_memory_provider_test.go	Resolve now checks that both the ContextEvaluator pointer and the function it references are non-nil before calling it; a new test confirms the flag falls back to its DefaultVariant with StaticReason instead of panicking when given a non-nil pointer to a nil function.

Estimated code review effort: 1 (Trivial) | ~5 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the nil ContextEvaluator guard fix in memprovider Resolve.
Description check	✅ Passed	The description matches the code change and regression test for the nil ContextEvaluator case.
Linked Issues check	✅ Passed	The change implements #521 by checking the underlying func for nil and falling back to the default variant.
Out of Scope Changes check	✅ Passed	The PR only touches the bug fix and its regression test, with no apparent unrelated changes.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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