fix(tsconfig): treat a missing extends target as non-fatal

[webpro]

Summary

With tsconfig: 'auto' (or an explicit config), oxc-resolver returns no result for every specifier under a tsconfig whose extends target can't be resolved — a package that isn't installed, or a generated config that doesn't exist yet (e.g. Nuxt's .nuxt/tsconfig*.json before nuxt prepare, or any config on a fresh clone before deps are installed).

tsc / typescript-go report TS6053 ("File … not found") and keep resolving with the options that did parse. This makes oxc-resolver match that.

Closes #1185.

Change

Skip a missing extends base and carry on, rather than failing:

• a bare-package base that resolves to TsconfigNotFound, and
• a missing relative target.

Still fatal / unchanged: unreadable (IOError) and malformed (Json) bases; circular extends; missing references.

Reproduction

// tsconfig.json
{ "extends": "@acme/not-installed/tsconfig.json" } // or "./.nuxt/tsconfig.app.json" before `nuxt prepare`

// a.ts
import { b } from './b'; // plain relative — does not depend on the tsconfig

resolve_file("a.ts", "./b") with tsconfig: 'auto':

• before: Err(TsconfigNotFound("@acme/not-installed/tsconfig.json"))
• after: Ok(".../b.ts") — matching tsc/tsgo

Tests

• New: test_extend_package_not_found_still_resolves, test_extend_relative_not_found_still_resolves.
• Flipped to the new behavior: test_extend_tsconfig_not_found and the undefined-#-import case in test_extend_imports (now Ok); the extends-not-found row in tsconfig_paths (now NotFound("ts-path")).
• Excluded extends-not-found from the tsconfck parse/invalid set — tsconfck classifies a missing extends as invalid; tsc/tsgo don't.

This is independent of the extends-inherited include/rootDirs fix (separate PR) and does not touch the references path.

[Boshen]

This is a hard one, I'll get to this soon.

[webpro]

Opened the PR because went through a few rabbit holes this morning, Claude still had good context and figured a PR might help. No hurries from our end.

For context: got triggered by webpro-nl/knip#1778 and pushed a workaround over there. If patches ship with oxc-resolver, I've verified we can remove this: webpro-nl/knip@91d6999