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 .NET local evaluator treated a present null value as not set and could not resolve is_not_set when the key was present. This could disagree with remote flag evaluation.

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.

💚 How did you test it?

• dotnet test tests/UnitTests/UnitTests.csproj --filter FullyQualifiedName~LocalEvaluatorTests
• dotnet build tests/UnitTests/UnitTests.csproj --configuration Release --no-restore --nologo
• bin/fmt --check
• pnpm changeset status (patches PostHog and PostHog.AspNetCore)
• Autoreview completed on commit 2f93275 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 change. The bundled autoreview tool reviewed the final committed diff against origin/main.

[github-actions]

posthog-dotnet Compliance Report

Date: 2026-08-26 09:52:24 UTC
Duration: 4426ms

✅ All Tests Passed!

17/17 tests passed

---

Feature_Flags Tests

✅ 17/17 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	246ms
Request Payload.Flags Request Uses V2 Query Param	✅	130ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	113ms
Request Payload.Flags Request Omits Authorization Header	✅	112ms
Request Payload.Token In Flags Body Matches Init	✅	112ms
Request Payload.Groups Round Trip	✅	115ms
Request Payload.Groups Default To Empty Object	✅	112ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	113ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	112ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	112ms
Request Lifecycle.No Flags Request On Init Alone	✅	5ms
Request Lifecycle.No Flags Request On Normal Capture	✅	116ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	217ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	116ms
Retry Behavior.Retries Flags On 502	✅	1117ms
Retry Behavior.Retries Flags On 504	✅	1115ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	214ms

[greptile-apps]

_{Reviews (1): Last reviewed commit: "chore: include ASP.NET Core in changeset" | Re-trigger Greptile}