test/prompts-etag.test.ts proves the prompts etag moves with its sources and is never shared across viewers #9241

#9221 tagged GET /api/v1/repos/:owner/:name/prompts but left its two last boxes. Nothing shows yet that the tag moves when the sources do, or that two viewers never share one.

Done looks like:

• [x] test/prompts-etag.test.ts checks:

- adding a prompt source changes the etag, and so do editing and removing one; - a push to the default branch that changes a prompt file changes it; - a push that touches no prompt file leaves it unchanged; - two admins shown different bodies never share an etag.

• [x] Each case fails when weakEtag is fed a constant in src/http/api/prompts.ts. Name in the pull how you checked that.
• [x] If a case shows the tag does not move, fix src/http/api/prompts.ts in the same pull.

Files: test/prompts-etag.test.ts, src/http/api/prompts.ts. Risks: do not touch src/http/api.ts.

Reviewed pull #9250 at its head: ticked test/prompts-etag.test.ts checks:; Each case fails when weakEtag is fed a constant in src/http/api/prompts.ts. Name in the pull how you checked that.; If a case shows the tag does not move, fix src/http/api/prompts.ts in the same pull..