feat(ai-observability): route eval-report agent through the ai-gateway

[brandonleung]

Problem

The eval-report agent built its OpenAI client directly on OPENAI_API_KEY. Make it routable through the internal Go ai-gateway by env, with no behavior change until flipped.

Changes

• The report agent calls build_openai_chat_client directly (the shared helper the cluster-labeling agents use): routes through the gateway when AI_GATEWAY_URL + AI_GATEWAY_API_KEY are set, else direct to OpenAI. The Cloud/DEBUG guard lives in the helper.

Rollout

Both vars unset (default) is today's behavior; setting them is the cutover. Two cutover-time items, neither active until then: confirm the gateway routes gpt-5.2, and drop the posthoganalytics callback (the gateway captures $ai_generation itself, so keeping it would double-capture).

Not in this PR

OpenAI summarization migration, stacked on top (#65044).

How did you test this code?

Agent-authored. The routing logic is covered by the helper's suite (test_llm_endpoint.py); the report-agent graph suite stays green. ruff clean. No manual run.

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Built with Claude Code; mirrors the merged cluster-labeling routing change. Skills: /branch-review and /pr-descriptions. Addressed Greptile's review (removed the superfluous _get_llm wrapper, calling the helper directly). Requires human review.

[greptile-apps]

Prompt To Fix All With AI

Fix the following 1 code review issue. Work through them one at a time, proposing concise fixes.

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### Issue 1 of 1
posthog/temporal/ai_observability/eval_reports/report_agent/graph.py:34-41
**Superfluous wrapper function**

`_get_llm` is now a single-line delegation to `build_openai_chat_client` with an identical signature and no added logic. By simplicity rules 3 (OnceAndOnlyOnce) and 4 (no superfluous parts), the wrapper should be deleted and the call site on line 196 updated to call `build_openai_chat_client(EVAL_REPORT_AGENT_MODEL, EVAL_REPORT_AGENT_TIMEOUT)` directly — eliminating one indirection and making the import the single statement of intent.

_{Reviews (1): Last reviewed commit: "feat(ai-observability): route eval-repor..." | Re-trigger Greptile}