TypeScript runtime tests fail in CI since ~April: ts-node incompatible with typescript@latest (TS 7) #4962

Every build (*, typescript) CI job currently fails, on every PR that triggers them. Each test dies at the Execute stage, before any ANTLR code runs, with:

TypeError: Cannot read properties of undefined (reading 'fileExists')
    at readConfig (.../ts-node/dist/configuration.js:91:33)

Cause

The test harness installs its toolchain unpinned (TsNodeRunner.java):

Processor.run(new String[] {NPM_EXEC, "--silent", "install", "-g", "typescript", "ts-node", "webpack", "webpack-cli"}, null);

npm's typescript@latest is now the TypeScript 7 (Go-based) rewrite, which no longer exposes the ts.sys internal API that ts-node 10.9.x (unmaintained since 2022) reaches into — ts-node crashes on startup before compiling anything.

Evidence it's the toolchain, not the tree

• dev tip 7d5770395 passed all three typescript jobs on 2026-02-16 (run 22078029856); the identical tree fails them today.
• The failure appears on every full-matrix PR regardless of content: carry trailing high surrogate across CodePointBuffer appends #4943, Fix NullPointerException in Antlr4MojoTest by downgrading Maven API to 3.6.3 #4947, Add automated documentation link validation #4949, Cpp: make DFAState edge lookups lock-free to speed up lexing #4953, Cpp: move DFA write locks from the ATN onto the DFA #4954, Fix typo: MY_CXX_WARNING_FLGAS -> MY_CXX_WARNING_FLAGS in Cpp CMakeLists.txt #4961. PRs that look green (e.g. reject tokenVocab names containing a path separator #4957–restrict deserialized types when reading maven dependency status #4960) simply didn't trigger the typescript jobs, so the breakage is easy to miss.
• Reproduced locally with the harness setup (ESM package.json + helper tsconfig + a test file importing antlr4): ts-node under typescript@7.0.2 fails with exactly the CI error; the same file runs fine under an executor that doesn't use the TypeScript compiler API.

Fix options

1. Swap ts-node for tsx (esbuild-based, no dependency on TypeScript's compiler API, actively maintained) — structurally immune to TypeScript's release cadence. PR to follow.
2. Pin the install (typescript@5 ts-node@10.9.2) — also goes green today, but freezes the tests to an aging TypeScript under an unmaintained runner.

Diagnosis developed with AI assistance (Claude Fable 5); I reviewed and reproduced it.