Add TraceContract compatibility fixtures for schema 0.1, 0.2, and 1.0

[rajudandigam]

Contribution lane: testing / fixtures
Difficulty: advanced
Ownership: community-owned
Priority: p1
Milestone: Evidence v2 & CI
Labels: testing, fixtures, area:core, community-owned, status:ready, difficulty:advanced, priority:p1, support:beta
Baseline: agent-inspect@6.17.1 → 6.18.0 niche launch

Problem

TraceContract needs explicit compatibility fixtures across schema 0.1, 0.2, and 1.0, including nested LLM metadata regressions related to #200.

Why it matters

1.x compatibility and check/matchers must not silently break older traces during 6.18.

Proposed scope

• Add fixtures for schema 0.1 / 0.2 / 1.0 TraceContract scenarios.
• Include nested metadata.model / LLM field cases covering check --allowed-model cannot see the LLM model on v0.1 traces (resolves the step name instead) #200 regressions (even if check --allowed-model cannot see the LLM model on v0.1 traces (resolves the step name instead) #200 remains open until fix(checks): resolve LLM model, provider, finish reason from nested metadata #201 merges).
• Assert deterministic pass/fail expectations.

Out of scope

• Destructive migrations
• Fabricating third persisted models
• Closing check --allowed-model cannot see the LLM model on v0.1 traces (resolves the step name instead) #200 in this issue

Suggested files

• TraceContract fixtures/tests
• Related check fixtures

Acceptance criteria

• Fixtures cover 0.1, 0.2, and 1.0
• Nested LLM metadata cases included
• Focused tests pass

Validation commands

pnpm exec vitest run <relevant>
git diff --check

Privacy / network notes

Synthetic traces only.

Contributor instructions

Comment first. Coordinate with #200/#201 status.

Maintainer-review boundary

Do not rewrite #200; this issue absorbs regression fixture needs.