fix(3597): pin shell:bash on npm ci/build steps in test.yml + regression test (#3673)

After PR #3649 (merge 40a442b2), test (windows-latest, 24) failed at npm ci with zero stdout/stderr (16s opaque exit). Identical code passed on the PR. Root cause: the npm ci and npm run build:sdk steps in .github/workflows/test.yml (jobs test and coverage) lacked an effective shell — neither step-level nor via defaults.run.shell. On windows-latest, Actions defaults to pwsh; npm.cmd → node.exe → npm-cli.js child-process chain under pwsh can swallow stderr.

Pin shell: bash on the 4 outlier steps. Add tests/workflow-shell-pinning.test.cjs that scans every workflow file referencing windows-latest, computes effective shell with proper workflow- and job-level defaults.run.shell inheritance, and fails CI if any run: npm|npx … step lacks an effective shell.

Closes #3672