test(server): add wire snapshots

[dustinbyrne]

💡 Motivation and Context

Add deterministic wire-format snapshot coverage for the Java server SDK as part of the coordinated server-side SDK snapshot rollout. These tests make it easier to compare capture, identity, exception, safe-property, and feature-flag request payloads across SDK implementations and catch accidental serialization changes.

The snapshots cover the existing behavior only; this test-only change does not alter the published SDK or require a release entry.

💚 How did you test it?

• ./gradlew :posthog-server:test --tests "com.posthog.server.PostHogServerWireSnapshotTest"
• make checkFormat

📝 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)

This PR was prepared with the Pi coding agent (worker). The human directed submission of the completed, independently reviewed server snapshot tests. The implementation keeps the change test-only and omits a changeset because it does not change released behavior. A human review is still required before merge.

[github-actions]

⚠️ `posthog-server` is modified but this PR has no changeset

This is informational — the PR is not blocked. Click the triangle above to collapse, or push a fix and this comment will auto-delete.

Modified in this PR but no changeset added:

• posthog-server

If this change should ship, run pnpm changeset and select a bump level.
If it isn't user-facing (refactor with no behavior change, internal tooling, generated files), no action needed.

[greptile-apps]

Prompt To Fix All With AI

### Issue 1
posthog-server/src/test/java/com/posthog/server/PostHogServerWireSnapshotTest.kt:183
**Remove the unused mock response**

`createClient` enqueues a second response even though each isolated test sends and records exactly one request. This obscures the expected request count and suggests an initialization request that never occurs.

```suggestion

```

---

For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

_{Reviews (1): Last reviewed commit: "test(server): add wire snapshots" | Re-trigger Greptile}

[greptile-apps]

_{Reviews (2): Last reviewed commit: "test(server): remove unused mock respons..." | Re-trigger Greptile}