fix(tsconfig): optimize TsconfigPathsPlugin and fix extends resolution bugs - #483

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**Summary**

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problem does the pull request solve? -->
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Fixes #706.

`TsconfigPathsPlugin` builds `extends` paths by hand. It appends `.json`
to any value without `.json` in it, and maps package names to
`node_modules` paths itself. tsc treats `./`, `../` and absolute values
as files (`.json` optional) and resolves everything else like a
`require`. So four forms that tsc accepts fail the whole tsconfig,
including the project's own `paths`.

This keeps the file branch and resolves everything else with a child
resolver set up like tsc's config lookup: `exports` with the
`require`/`types`/`node` conditions, the `tsconfig` field, then
`tsconfig.json`. That replaces the package special cases added in #483,
#575 and #631. Like tsc, backslashes are normalized first. When the
lookup fails, a bare file name such as `"tsconfig.base.json"` still
loads from the config's directory, as it always has here, though tsc
rejects it.

Resolving `@/foo` with the #706 repro, where tsc gives `src/foo.ts` for
all four:

| `extends` | Before | After |
| ---------------------------------- |
------------------------------------------ | ------------ |
| `"cfgpkg/strict"` via `exports` | `ENOENT …
node_modules/cfgpkg/strict.json` | `src/foo.ts` |
| `"cfgpkg"` with a `tsconfig` field | `ENOENT …
node_modules/cfgpkg/tsconfig.json` | `src/foo.ts` |
| `"./configs/base.tsconfig"` | `ENOENT … configs/base.tsconfig.json` |
`src/foo.ts` |
| `".."` | `ENOENT … dotdot/pkg/...json` | `src/foo.ts` |

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**What kind of change does this PR introduce?**

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fix

**Did you add tests for your changes?**

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merge your changes unless you add tests. -->

Yes: five cases in `test/tsconfig-paths.test.js`, with fixtures in
`test/fixtures/tsconfig-paths/extends-resolution/`. The four #706 forms
fail on `main`. The `..` case also checks that a sibling `<dir>.json`
doesn't win. The bare file name case passes on `main` and guards the
kept fallback.

**Does this PR introduce a breaking change?**

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and a migration path for existing applications. -->

No.

**If relevant, what needs to be documented once your changes are merged
or what have you already documented?**

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n/a

**Use of AI**

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Yes. I used Claude Code to write the change and the tests. I reviewed
every line, checked each case against tsc's `getExtendsConfigPath`, and
ran `npm run lint` and the full test suite locally.


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

<!-- coderabbit-ai-generated:v1 -->

## Summary by CodeRabbit

* **Bug Fixes**
* TypeScript configuration inheritance now follows TypeScript-style path
resolution, including package exports, package configuration fields,
extensionless config files, and parent-directory references.
* Unsupported bare filenames are no longer treated as valid
configuration paths.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->