test: validate OpenClaw placeholder replay exports

[mnajafian-nv]

Overview

This PR adds exporter-visible validation for OpenClaw placeholder replay fallback behavior across ATIF, ATOF, and OpenInference.

• I confirm this contribution is my own work, or I have the right to submit it under this project's license.
• I searched existing issues and open pull requests, and this does not duplicate existing work.

Details

• Adds ATIF validation that OpenClaw placeholder replay preserves an empty user step while keeping the raw replay request in Step.extra.llm_request, including placeholderRequest.
• Adds ATOF validation that OpenClaw placeholder replay lifecycle payloads are preserved as raw JSONL without inventing higher-fidelity structure.
• Adds OpenInference validation that OpenClaw placeholder replay falls back to sanitized JSON input.value / input.mime_type and does not invent flattened llm.input_messages.* attributes.
• Keeps the change test-only and focused on exporter-visible proof of hook-backed OpenClaw fallback behavior.
• Avoids runtime, adapter, provider-event, or exporter implementation changes in this PR.

Validated with:

• cargo test -p nemo-relay atif
• cargo test -p nemo-relay atof
• cargo test -p nemo-relay openinference
• cargo fmt --all --check
• uv run pre-commit run --files crates/core/tests/unit/atif_tests.rs crates/core/tests/unit/observability/atof_tests.rs crates/core/tests/unit/observability/openinference_tests.rs

Where should the reviewer start?

Start in crates/core/tests/unit/observability/openinference_tests.rs for the fallback-shape assertion, then review crates/core/tests/unit/atif_tests.rs for the ATIF empty-user-step and raw-request proof, and crates/core/tests/unit/observability/atof_tests.rs for the raw JSONL preservation check.

Related Issues: (use one of the action keywords Closes / Fixes / Resolves / Relates to)

• Relates to OpenClaw observability consistency work.