[Sampling Replay 5/8] Preserve sampling support through session rollouts

[nanjiangwill]

Stack position

Part 5/8 of the bounded sampling-support replay series. Merge after #3349. #3321 is merged in main.

Layer-only diff: nanjiangwill/miles@feat/sampling-replay-rollout-capture...feat/sampling-replay-sessions

Review umbrella: #3354.

Summary

• Carry bounded-sampling request defaults through session server v1 and v2.
• Retain native support metadata in server-owned records while stripping it from client responses.
• Reconstruct supports and native rollout log probabilities during sample collection.
• Extend the typed session codec field selection without changing the disabled path.
• Support agentic and tool-call rollouts, including explicit evaluation opt-out.

Why this layer is separate

Session rollouts have a distinct lifecycle: requests are proxied, metadata is stored server-side, turns may branch or be truncated, and samples are encoded later. Keeping this layer separate lets reviewers verify that lifecycle without re-reviewing trainer math or direct SGLang generation.

Validation

Focused tests cover v1 and v2 configuration, request resolution, record conversion, truncation, aborts, client metadata stripping, codec selection, agentic collection, and router integration.

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[nanjiangwill]

Closing this layer-specific review PR in favor of the single integrated sampling-replay PR #3354. The implementation and validation remain available there.