fix(flags): retry flag requests on transient network errors

[marandaneto]

💡 Motivation and Context

/flags/?v=2 evaluation should tolerate transient network failures, but should not retry HTTP/API responses such as 408, 429, or 5xx.

This PR adds bounded retry/backoff only for transient network failures (timeouts, connection resets/lost connections, and EOF-style failures) and keeps HTTP/API status errors terminal.

💚 How did you test it?

See the repo-specific command below.

📝 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)

Implemented by pi after a maintainer-directed cross-SDK pass. The chosen policy is transient network-only retry/backoff for /flags/?v=2; HTTP/API statuses remain non-retryable by design. Connection-refused failures also fail fast where the platform exposes that distinction.

Tested with:

dotnet test tests/PostHog.Unity.Tests/PostHog.Unity.Tests.csproj --filter NetworkClientTests

[greptile-apps]

_{Reviews (1): Last reviewed commit: "fix: retry feature flag requests on netw..." | Re-trigger Greptile}

[greptile-apps]

_{Reviews (2): Last reviewed commit: "Merge branch 'main' into pi-flags-networ..." | Re-trigger Greptile}