Play Framework

Revert "feat(testrunner): parallelize unit tests across classes via VT pool"

Squashes the revert of two commits:
  - e6df0b955 feat(testrunner): split DB-touching unit tests onto a single-permit lane
  - 40928be0c feat(testrunner): parallelize unit tests across classes via VT pool

Restores FirePhoque to a single sequential WebClient loop and TestRunner.list
to flat unprefixed entries. The U:/D:/F:/S: protocol, the ASM-based DB
classifier in TestRunner, and the dual-lane VT executor in FirePhoque are
all gone.

The framework-side TestEngine.functionalTestsExecutor change (now VT-backed
single-threaded) and the Play.java warn-block extension stay — they were
shipped under separate commits and are independent of FirePhoque parallelism.

v1.12.3 on GitHub still ships the parallel implementation; the next release
will be the first non-parallel one.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>