Align Base CI on one immutable base-bash-libs GA revision

[codeforester]

Summary

Base's CI currently mixes the published base-bash-libs v2.0.0 GA commit with the older v2.0.0-rc.1 commit.

Evidence

• The BATS job checks out GA commit b4243765726c133499feeabdc50154f99c0fec12.
• The macOS, integration, and Ubuntu source-checkout jobs still check out RC commit c134fb8a3397e2cfee1d90845cec44f56dacae7b.
• The changelog says CI and source-checkout validation consume the published GA v2.0.0.
• This regresses the contract established by chore(release): promote base-bash-libs v2.0.0 GA pin #1935/ci: promote base-bash-libs v2.0.0 GA pin #1936, which promoted all Base pins to the immutable GA revision.

Impact

Different jobs validate different base-bash-libs APIs, so a change can pass one CI path while exercising an obsolete dependency on another. That weakens release confidence and makes the source-checkout contract misleading.

Acceptance criteria

• Every active base-bash-libs checkout in .github/workflows/tests.yml uses the exact GA commit.
• A workflow-contract test or equivalent assertion prevents the RC commit from returning.
• Full local and hosted validation passes, with the release evidence recorded in the issue.