fix(flags): align presence operator semantics

[marandaneto]

💡 Motivation and Context

sdk-specs PR #49 defines presence operators using property-key presence and partial property context. The posthog-server evaluator treated a present null value as not set and could not resolve is_not_set when the key was present.

This change makes every present key match is_set and not match is_not_set, including null and other falsey values. An omitted key remains inconclusive so remote evaluation stays eligible. Group flags now select the property map for their matching group type before applying the same rules.

💚 How did you test it?

• ./gradlew :posthog-server:test --tests "com.posthog.server.internal.FlagEvaluatorTest"
• ./gradlew :posthog-server:test --tests "com.posthog.server.internal.PostHogFeatureFlagsTest"
• ./gradlew spotlessCheck
• Autoreview completed on commit 6811c125 with no actionable findings.

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran pnpm changeset to generate a changeset file

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Pi coding agent worker subagents implemented and validated the focused posthog-server change. The bundled autoreview tool reviewed the final committed diff against origin/main.

[greptile-apps]

_{Reviews (1): Last reviewed commit: "fix(flags): use group type properties fo..." | Re-trigger Greptile}