fix(parity): surface Cargo failures before reporting regressions

[proggeramlug]

Fixes #10920.

• Show the last 40 lines of Cargo output when either archive build fails.
• Fail before running the mixed parity suite when Cargo is unavailable; that suite re-enables auto-optimize for ext-routed fixtures even with prebuilt archives.

Verification: bash -n run_parity_tests.sh, git diff --check, and targeted mock-Cargo runs covering a failed build and a prebuilt mixed-suite run without Cargo on PATH.

Summary by CodeRabbit

• Bug Fixes

  • Parity test runs now clearly report Cargo build failures and stop when Cargo is unavailable.
  • Build failures provide the final log output, making errors easier to diagnose.
  • Prevents missing Cargo installations from being incorrectly reported as runtime or link regressions.

• Documentation

  • Added changelog coverage for improved parity harness build-error handling.

[coderabbitai]

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Advanced

Run ID: 6c84a63d-6838-4ba1-80b0-4f341608eff4

📥 Commits

Reviewing files that changed from the base of the PR and between a022cf2 and 4761e41.

📒 Files selected for processing (2)

• changelog.d/10963-parity-harness-build-errors.md
• run_parity_tests.sh

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📝 Walkthrough

Walkthrough

The parity harness now checks for Cargo before applicable runs and reports the last 40 lines from failed compiler, runtime, and WebAssembly host builds. A changelog entry documents the behavior.

Changes

Parity build diagnostics

Layer / File(s)	Summary
Cargo availability preflight run_parity_tests.sh	The harness exits with an error when Cargo is unavailable and builds or auto-optimization can run.
Build failure logs run_parity_tests.sh, changelog.d/10963-parity-harness-build-errors.md	The release and WebAssembly host builds write log files and print their last 40 lines on failure. The changelog records the updated behavior.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix · Severity of issue fixed: Medium

Merge Risk: ⚪ Minimal · up to 4761e

Parity runs now report required Cargo failures clearly and expose relevant build diagnostics; the change is ready to merge.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the main change: exposing Cargo failures before the parity harness reports regressions.
Description check	✅ Passed	The description provides the change summary, concrete updates, linked issue, and verification steps. It does not use the template headings or include the checklist, but the required information is mos…
Linked Issues check	✅ Passed	Issue #10920 requires Cargo build diagnostics and an environment failure when Cargo is unavailable before the mixed parity suite runs. run_parity_tests.sh now captures both archive-build outputs in …
Out of Scope Changes check	✅ Passed	The changes stay within Issue #10920. The script changes add build-error reporting and the Cargo precondition. The changelog fragment documents those changes. No unrelated production behavior or publi…
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 1…

✨ Finishing Touches 💡 1

🧪 Generate unit tests (beta)

• Create a new PR

🛠️ Fix failing CI checks 💡

• Commit to this branch
• Create a new PR

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[proggeramlug]

Landed on main in merge train 256 (#11018, v0.5.1638), main f5cfbff882.

Carried at head 4761e41636. The landed tree is byte-identical to the validated train tree (9b108dd3c9), and CI on the train head passed every job except the known public-baseline lint step: all 6 gap shards, cargo-test, e2e-scoped, gc-stress, check, warnings and security-audit green.

Trains rebase-merge, so commits get new SHAs and GitHub cannot mark this PR merged. Closed as landed.