Fix invalid Cloud Run candidate revision tags

[anth-volk]

Problem

The first post-merge simulation deployment generated the Cloud Run revision tag s1. Cloud Run rejected the candidate because revision tags must contain at least three characters. The production prefix would have generated the similarly invalid tag p1.

Scope

• Use valid, environment-specific Cloud Run revision tag prefixes for beta and production.
• Add regression coverage for the first workflow run, when the numeric suffix is shortest.

Acceptance criteria

• Beta and production candidate tags satisfy Cloud Run's length and naming constraints starting at workflow run 1.
• The entrypoint deployment tests cover this boundary case.
• The affected GitHub Actions workflows pass actionlint.