fix: resolve tsconfig extends like tsc - #709

#711)

**Summary**

Fixes #705.

#709 fixed `extends` lookup (#706), but configs loaded through `extends`
still resolve `paths` and `baseUrl` differently from tsc. This PR builds
on #710 by @bardrop. It applies its two commits onto current `main`,
since #710 now conflicts with #709 in `test/tsconfig-paths.test.js`. It
also adds two fixes:

- **#710 (commits `f70894d`, `5520785`)**: `paths` without `baseUrl`
resolve relative to the config that declares them (`pathsBasePath`).
`${configDir}` in an inherited `baseUrl` becomes the root config
directory. An inherited empty `baseUrl` becomes the directory of the
config that declares it.
- **Empty root `baseUrl` (`99da949`)**: `"baseUrl": ""` was used for
`paths` but never added to `modules`, so bare requests did not resolve
from it. This bug also exists on `main`.

  ```diff
- if (baseUrl && absoluteBaseUrl && !modules.includes(absoluteBaseUrl))
{
  + if (baseUrl !== undefined && !modules.includes(absoluteBaseUrl)) {
  ```

- **Generated types (`9ee297e`)**: regenerates
`types/TsconfigPathsPlugin.d.ts` for the new `pathsBasePath` JSDoc
property. Without it, CI's `build:types` check fails.

I compared enhanced-resolve with `ts.resolveModuleName` (TypeScript
6.0.3, `moduleResolution: bundler`) on 34 setups. They cover the #705
and #706 repros, multi-level and array `extends`, packages (`exports`,
the `tsconfig` field, scoped), `${configDir}`, empty `baseUrl`,
backslashes, and project references.

| Code                   | Setups that differ from tsc |
| ---------------------- | --------------------------- |
| `main`                 | 16 / 34                     |
| `main` + #710          | 1 / 34 (empty root `baseUrl`) |
| this PR                | 0 / 34                      |

The #705 repro, before and after:

```text
before: one @/foo -> Can't resolve '@/foo'   two @/foo -> Can't resolve '@/foo'   three foo -> Can't resolve 'foo'
after:  one @/foo -> src/foo.ts              two @/foo -> src/foo.ts              three foo -> src/foo.ts
```

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

fix

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

Yes. #710 adds 6 tests with fixtures in
`test/fixtures/tsconfig-paths/extends-paths-base/`. I added one test
that resolves `src/foo` through an empty root `baseUrl`; it fails
without `99da949` and passes with it. `npm run lint` is clean and `npm
run test:only` passes (1664 tests).

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

No.

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

n/a. The changeset from #710 is included.

**Use of AI**

Yes. I used Claude Code to apply #710 onto `main`, write a differential
script against tsc, find and fix the remaining empty-`baseUrl` case, add
its test, and regenerate the types. Lint and the full test suite were
run locally.

---
_Generated by [Claude
Code](https://claude.ai/code/session_013FhutR7wqtULZXURwFF5ER)_

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
<!-- coderabbit-ai-generated:v1 -->

## Summary by CodeRabbit

* **Bug Fixes**
* Corrected resolution of inherited TypeScript path aliases when
`baseUrl` is missing or empty.
* Improved handling of `${configDir}` in inherited `baseUrl` settings
and path resolution across extended configurations.
* Prevented one extended configuration from affecting subsequent loads
that share a filesystem cache.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Mradul Singh <msingh@dealmeridian.com>
Co-authored-by: Claude <noreply@anthropic.com>

This PR was opened by the [Changesets
release](https://github.com/changesets/action) GitHub action. When
you're ready to do a release, you can merge this and the packages will
be published to npm automatically. If you're not ready to do a release
yet, that's fine, whenever you add more changesets to main, this PR will
be updated.


# Releases
## enhanced-resolve@5.26.2

### Patch Changes

- Export the `UnsafeCache` type of the `unsafeCache` option. (by
[@alexander-akait](https://github.com/alexander-akait) in
[#703](#703))

- Resolve tsconfig `extends` the way tsc does, including package
`exports`. (by [@abrahamguo](https://github.com/abrahamguo) in
[#709](#709))

- Resolve `paths` inherited through `extends` (with no `baseUrl`)
relative to the tsconfig that declares them, substitute `${configDir}`
in an inherited `baseUrl`, treat an empty `baseUrl` as the directory of
its tsconfig, like tsc, and stop extending a tsconfig from changing it
for later loads that share the file system cache. (by
[@alexander-akait](https://github.com/alexander-akait) in
[#711](#711))

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>