test(test-selection): pin the degradation on a selection we predate

[AlexandreGaubert]

The engine is gaining a third answer on the test-selection endpoint --
selection: "empty", meaning "run no test, the predecessor already ran
them and they passed" (Mergifyio/monorepo#39373). Every published client
predates it, and each degrades correctly today only by accident of three
unpinned facts: the decoder types selection as a free String,
test_selection_dict always sets the tests key, and
TestSelection.__post_init__ normalises anything that is not a non-empty
subset back to a full run. Tighten selection into a closed enum for
hygiene -- the obvious next edit -- and every already-shipped client stops
decoding any answer the server invents.

Pin the two ends of that property: decoding an empty payload in
mergify-ci-api, and normalising an unrecognised selection to "run
everything" rather than "run nothing" in the plugin. Both were verified to
fail under the edit they guard against. The marshalling comment claiming a
missing tests means a full answer is corrected -- it holds for any
answer carrying no subset.

pytest only: vitest and playwright share the decoder but normalise in
their own TypeScript, and the pilot does not enable them.

Related to MRGFY-8931

[mergify]

Merge Protections

🟢 All 6 merge protections satisfied — ready to merge.

Show 6 satisfied protections

🟢 🤖 Continuous Integration

• all of:
  • check-success=ci-gate

🟢 👀 Review Requirements

• any of:
  • #approved-reviews-by>=2
  • author = dependabot[bot]
  • author = mergify-ci-bot
  • author = renovate[bot]

🟢 Enforce conventional commit

Make sure that we follow https://www.conventionalcommits.org/en/v1.0.0/

• title ~= ^(fix|feat|internal|docs|style|refactor|perf|test|build|ci|chore|revert|ui)(?:\(.+\))?!?:

🟢 🔎 Reviews

• #changes-requested-reviews-by = 0
• #review-requested = 0
• #review-threads-unresolved = 0

🟢 📕 PR description

• body ~= (?ms:.{48,})

🟢 🚦 Auto-queue

When all merge protections are satisfied, this pull request will be queued automatically.

[mergify]

Merge Queue Status

• ✅ Entered queue — 2026-08-31 12:34 UTC · Rule: default · triggered by merge protections
• ✅ Checks passed · on draft merge queue: checking #122 on main (41d7cdf) #133
• ✅ Merged — 2026-08-31 12:38 UTC · at 98cbb5f5f516262042a9ab6cc0c031d109d3fe45 · squash

This pull request spent 3 minutes 58 seconds in the queue, including 3 minutes 1 second running CI.

Required conditions to merge

• all of [🛡 Merge Protections rule Enforce conventional commit]:
  • title ~= ^(fix|feat|internal|docs|style|refactor|perf|test|build|ci|chore|revert|ui)(?:\(.+\))?!?:
    • test(test-selection): pin the degradation on a selection we predate #122
• all of [🛡 Merge Protections rule 👀 Review Requirements]:
  • any of:
    • #approved-reviews-by>=2
      • test(test-selection): pin the degradation on a selection we predate #122
    • author = dependabot[bot]
      • test(test-selection): pin the degradation on a selection we predate #122
    • author = mergify-ci-bot
      • test(test-selection): pin the degradation on a selection we predate #122
    • author = renovate[bot]
      • test(test-selection): pin the degradation on a selection we predate #122
• all of [🛡 Merge Protections rule 📕 PR description]:
  • body ~= (?ms:.{48,})
    • test(test-selection): pin the degradation on a selection we predate #122
• all of [🛡 Merge Protections rule 🔎 Reviews]:
  • #changes-requested-reviews-by = 0
    • test(test-selection): pin the degradation on a selection we predate #122
  • #review-requested = 0
    • test(test-selection): pin the degradation on a selection we predate #122
  • #review-threads-unresolved = 0
    • test(test-selection): pin the degradation on a selection we predate #122
• all of [🛡 Merge Protections rule 🤖 Continuous Integration]:
  • all of:
    • check-success=ci-gate