ci(tools): run-syntax self-test reads e6eac090, which the rewritten main no longer has #910

Goal

run-syntax fails on every PR that touches .github/workflows/**, whatever the PR changes. Make its self-test independent of the repository's history, without weakening it.

Where we are

• tools/test_check_run_syntax.py builds two cases from git show e6eac090:.github/workflows/build-matrix.yml and ...iddr-golden-diff.yml, the two defects the gate was written for. Both cases expect rc 1.
• main was rewritten after that commit. e6eac090 is no longer an ancestor of main (compare e6eac090...main: diverged, 117 ahead, 68 behind). The same tree is 78606c5d on today's main.
• A fresh clone does not have e6eac090, so git show returns nothing. The empty input gives rc 2, and both cases go BAD: 61 of 63 cases as expected (run 38010467066, job 114089022932, on fix(trinet): a coordinator restart no longer pays the same (node, nonce) again #907).

Boundary

• No changed expected rc, no dropped case, no disabled step, no baseline.
• Inputs are byte-identical to the blobs at e6eac090.

Success Criteria

1. run-syntax green on the PR head: 63 of 63 cases as expected, and Every run script parses green.
2. An edited or missing fixture makes its case BAD and names the fixture.

Dmitrii Fedorov (@dmitrii-f-t27)