test: continue full-suite compatibility fixes

[senamakel]

Summary

Carries the remaining full-suite compatibility fixes onto current main, including delegation context propagation and harness/session test-contract updates.

Dependency

Requires tinyhumansai/tinyagents#180 for the updated vendor/tinyagents gitlink.

Validation

• RUST_MIN_STACK=16777216 bash scripts/ci-cancel-aware.sh pnpm test:rust (remaining failures are still being addressed)
• Focused message conversion, parser, and delegation-context tests passed before rebasing onto current main.

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