fix(resolver): resolve tsconfig extends through the real path

[orgads]

Motivation

Closes #15717.

When resolve.tsConfig points at a config whose extends chain passes through
a package in node_modules, and that package is a symlink (the normal pnpm
layout), the build fails:

ERROR in ./entry.ts 1:1-31
  × Tsconfig not found @tsconfig/node22/tsconfig.json

The package extends specifier is resolved through node_modules, but the
resulting path was never canonicalized. The nested extends inside that
package was therefore looked up from the symlink location
(<root>/node_modules/shared-config/), which walks up to the project's root
node_modules, where the package's own dependencies do not exist — under pnpm
they live next to the real package directory in
node_modules/.pnpm/shared-config@…/node_modules/.

tsc resolves the same chain without complaint, since it looks up
module-resolved configs through their real path. The same tree materialized
without a symlink (npm-style) also builds fine today, confirming the problem is
symlink-specific.

This is easy to hit unintentionally: any monorepo shipping a shared base
tsconfig as a package that itself extends @tsconfig/* breaks under pnpm. It
also surfaces very indirectly through Rstest, which sets resolve.tsConfig
automatically and swallows the resolver error.

Changes

Apply the existing load_realpath to the config resolved for a bare extends
specifier, so nested extends (and relative extends within that package)
resolve from its real directory. options.symlinks is honored, so
symlinks: false keeps the previous behavior. Relative and absolute extends
specifiers are untouched — tsc only resolves module-resolved configs through
the real path.

Regression test: a pnpm-style fixture where node_modules/shared-config is a
symlink into a store directory holding its @tsconfig/base dependency next to
it, absent from the project's own node_modules. Without the fix the test
fails with exactly TsconfigNotFound("@tsconfig/base/tsconfig.json"). The
symlink is created at test runtime (skipped when unprivileged on Windows),
matching the existing symlink test.

🤖 Generated with Claude Code

[LingyuCoder]

@stormslowly PTAL