fix(store): restore default tsconfig in setFiles

[ifer47]

What changed

• Restore the default tsconfig.json whenever setFiles() receives no tsconfig.
• Reuse the same defaulting path during store initialization and file replacement.
• Keep an explicitly provided tsconfig.json unchanged.
• Add focused Node test-runner coverage through Vite's SSR module loader, without adding a dependency.

Why

setFiles() replaces the store's entire file record. Store initialization adds a default tsconfig.json, but the replacement path did not restore that default, so language tooling lost its configuration after callers replaced only their source files.

Fixes #383.

How tested

• pnpm test
• pnpm typecheck
• pnpm lint
• pnpm exec prettier --check src/store.ts test/store.test.mjs package.json
• pnpm build (passes; retains the existing API Extractor warning about its bundled TypeScript version)
• Vercel preview is unavailable for this fork because the check requires Vue organization deployment authorization; the local production build passes.

Risk and rollout

• Risk: Low. The only new runtime behavior is adding the existing default tsconfig when setFiles() omits it; caller-provided tsconfig content still wins.
• Rollback: Revert this commit to restore the previous replacement behavior.

Notes for reviewers

• Duplicate search covered open, closed, and merged PRs; open and closed issues; issue comments and linked PRs; commit history; remote branches; src/store.ts; and behavior/API terms.
• Search terms included 383, setFiles tsconfig, tsconfig.json, src/store.ts setFiles, and Using setFiles removes tsconfig.json.
• No PR or upstream branch covers Using setFiles removes tsconfig.json #383. The broad tsconfig.json search only found unrelated PRs chore(deps): update all non-major dependencies #251 (dependency updates) and feat: support ts import type modifiers on import names #28 (TypeScript import modifiers).
• Historical setFiles commits 177f675 and 424e00d predate the tsconfig feature; PR feat: add tsconfig file #114 added tsconfig support but did not restore it after setFiles().

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