fix(typescript): respect resolveHiddenExtensions when a host pre-installs resolveModuleNameLiterals

[NullVoxPopuli]

Summary

When any registered language plugin sets typescript.resolveHiddenExtensions: true, the wrapped moduleResolutionHost.fileExists in resolveModuleName.ts is what makes extensionless imports of plugin-extension files resolvable (it answers Foo.d.ts as true when Foo.<ext> exists on disk, so tsc's hard-coded tryAddingExtensions accepts the lookup).

The fast-path in proxyCreateProgram short-circuits a resolution batch back to the original host whenever no import literal ends in a plugin extension. In one-shot tsc the original host has no resolveModuleNameLiterals installed, so the condition is false and volar's wrapper runs.

But tsc --watch installs a cached resolver on the compiler host before volar's proxy runs:

// typescript/lib/_tsc.js (createWatchProgram)
compilerHost.resolveModuleNameLiterals = resolutionCache.resolveModuleNameLiterals.bind(resolutionCache);

With that resolver defined, extensionless imports now take the fast-path and bypass the wrapper. The host's plain fileExists doesn't know about the .<ext> ↔ .d.ts shim, so the import fails with TS2307 in watch mode while succeeding in one-shot mode.

This was hit by glint for .gts files: import Foo from './Foo' (where Foo.gts exists) works under ember-tsc but errors under ember-tsc --watch. Same shape as the TS Plugin mode limitation already tracked at typed-ember/glint#806.

Change

Skip the fast-path when any plugin opts in to resolveHiddenExtensions, matching the existing pluginExtensions check. Applied to both resolveModuleNameLiterals and resolveModuleNames.

Test plan

• Added packages/typescript/tests/proxyCreateProgram.spec.ts with two cases:
  • With resolveHiddenExtensions: true, extensionless imports must not be delegated to the host's resolveModuleNameLiterals.
  • Without resolveHiddenExtensions, the existing fast-path is preserved — extensionless imports still delegate to the host.
• pnpm build clean
• pnpm test — 72 passed (10 files), including the two new cases. Reverting the production change makes the first case fail, confirming it catches the regression.
• Verified end-to-end against the original repro in glint: with this patch applied to the @volar/typescript source and rebuilt, ember-tsc --watch against a project with extensionless .gts imports reports Found 0 errors (was TS2307: Cannot find module './Foo').