fix(server): tolerate overflowing logprobs in triplet queries

[ningyixi]

An oversized integer logprob (for example, 10**400) raises OverflowError during float() conversion, causing GET /api/rollouts/{id}/events?format=triplet to fail instead of marking the sample's logprobs unusable.

Handle OverflowError alongside the existing conversion errors. Add API-level regression coverage for both chat logprobs.content and completion logprobs.token_logprobs, checking that token IDs survive and response_log_probs is null.

Validation:

• Both regression cases fail with OverflowError before the fix.
• uv run --locked --no-sync pytest -q tests/server — 34 passed.
• Repository-wide Ruff lint, formatting checks, pre-commit hooks, and copyright-header checks passed.
• Pyright passed for the two changed files. The full VERL/CPU training suite was not run.

Prepared with Codex AI assistance; the diff and before/after regression results were checked locally. No model service or GPU is needed for these tests.

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[Copilot]

🟢 Approval recommended

The narrow fix follows existing error handling and includes regression coverage for both affected formats.

No open issues 🎉

What changed in this PR

Prevents overflowing logprob values from failing triplet event queries.

Changes:

• Handles OverflowError using the existing unusable-logprobs fallback.
• Adds API regression coverage for chat and completion formats, verifying token IDs survive and logprobs become null.

File	Description
tests/​server/​test_endpoints.py	Tests overflowing logprobs in both response formats.
agentlightning/​server/​routes/​events.py	Catches overflow during logprob conversion.

_{Review effort: Balanced}