fix(vscode): resolve @rstest/core from the rstack package for bridged projects

[fi3ework]

Summary

A folder driven through rstack's Rstest config shim (rstack.config.* with no native rstest.config.*) resolved @rstest/core by walking node_modules up from the config directory. Under pnpm's isolated layout a project that depends on rstack alone has no node_modules/@rstest/core — the core sits beside rstack in the virtual store — so every bridged project logged Cannot find "@rstest/core" and Rstest was unusable. Reproduced in a real VS Code against pnpm add rstack@0.6.1 with default pnpm settings.

The lint stack already anchors its @rslint/core walk-up at the resolved rstack directory (stacks/lint/resolution.ts); the test stack now does the same:

• resolveRstackShim returns the resolved rstack package directory, and the bridge threads it into RstestApi as the default @rstest/core / CLI-bin resolution anchor (rstestResolutionDir). Native projects keep the cwd anchor byte-for-byte; rstack.rstest.rstestPackagePath still overrides.
• A missing core is reported once: createChildProcess throws ReportedRstestResolutionError, which the project initializer no longer re-logs as Failed to initialize project config.
• E2E fixtures now install with pnpm's default isolated layout — the --config.publicHoistPattern=@rslint/core / @rstest/core flags in setupFixtures.mjs are removed. They hid this bug; and even without them the old walk-up escaped the fixture into this repo's own dev copy of @rstest/core, so the suite stayed green for the wrong reason. The rstest bridge suite now asserts the resolved core lives inside the fixture's node_modules; the lint bridge suite stages a project-visible @rslint/core (linked from the one rstack resolves) only for its native-ownership transition test.
• Unit tests pin the anchor per project kind (native / bridged / configured) against a .pnpm-shaped tree.

Related Links

• packages/vscode/AGENTS.md: the test × rstack.config.* bridge gotcha now records that bridged projects resolve @rstest/core from the rstack package directory, mirroring lint.

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).