test runner: true isolation for the web runner via a separate application context (successor to #3025)

[bpamiri]

Context

#3025 tracked the web test runner mutating the live application.wheels struct (swap → run → restore in vendor/wheels/tests/runner.cfm). Its titled approach — a request-scoped config overlay — was ruled infeasible by the adversarial analysis in that thread (blockers B1–B9: dialect adapter baked per model class at init, app-scoped model cache, ~24+ direct application.wheels.X reads bypassing $get, populate/migrator DDL outside any overlay, browser/parallel tests requiring the global swap). Blast radius of the full overlay ≈ 60–110 call sites.

PR #3373 shipped the maintainer-endorsed safe slice: the swap window is now serialized under an exclusive named lock (with ParallelRunner re-entrancy detection), and the orphaned RocketUnit twin (vendor/wheels/rocketunit_tests/Test.cfc) was deleted. #3025 was auto-closed by that PR's squash title; this issue carries the remainder.

Residual gap (what this issue tracks)

The lock serializes test-vs-test only. A normal request running concurrently with a test run still reads swapped (test) configuration for the duration of the run. True isolation requires the direction the #3025 analysis judged superior: running the test suite in a separate application context (own application name/scope, own model/dialect caches, own datasource wiring) so the live app's application.wheels is never touched.

Design questions to settle

• Bootstrapping a second Wheels application context inside the same servlet context (application name derivation, onApplicationStart reuse, teardown).
• How browser/parallel specs (fresh top-level HTTP requests) address the isolated context (dedicated URL prefix vs header-based routing).
• Migrator/populate DDL targeting the test datasource without global state.
• CLI runner (wheels test) parity — it uses a different chain than runner.cfm and must not regress.

References

• test runner: web runner mutates live application.wheels — replace global swap with request-scoped overlay #3025 (analysis with blockers B1–B9, 2026-06-22 comment)
• PR fix(test): serialize web test-runner config swap under a named lock (#3025) #3373 (safe slice: named-lock serialization + twin deletion)
• Running Wheels 4 in a subfolder - And figuring out tests (broken) #3344 / Running Wheels 4 in a subfolder - And figuring out tests (broken) #2887 (the original "they fight each other" user reports)