Running two ng serve instances (same project, different configurations) causes “504 Outdated Optimize Dep” errors in Angular CLI 20.3.8

### Command

serve

### Is this a regression?

- [x] Yes, this behavior used to work in the previous version

### The previous version in which this bug was not present was

20.3.6

### Description

Since updating to Angular v20.3.9 / CLI v20.3.8, it’s no longer possible to run two ng serve instances for the same project (e.g. localized builds) in parallel.
The second instance fails to load with a 504 (Outdated Optimize Dep) error for one or more chunks (e.g. chunk-5V7QYGNA.js).

This setup worked correctly in previous Angular 20.3.6 versions (before the CLI migrated the dev-server to the Vite-based implementation).

The issue appears to be caused by both ng serve processes sharing the same Vite optimizer cache directory inside .angular/cache/.../vite.
When one process updates or invalidates the cache, the other process detects stale metadata and refuses to serve pre-bundled chunks.

### Minimal Reproduction

package.json scripts
"scripts": {
"start": "ng serve --host 0.0.0.0",
"startfr": "ng serve -c fr --host 0.0.0.0",
"startfr-alt": "ng serve -c fr --host 0.0.0.0 --port 4201",
"start:both": "concurrently \"npm run start\" \"npm run startfr-alt\""
}

Steps to reproduce
1. Run npm run start:both.
2. Wait for both dev servers to start.
3. Observe that the second instance fails to load in the browser console with:
Failed to load resource: the server responded with a status of 504 (Outdated Optimize Dep)

### Exception or Error

```text
Failed to load resource: the server responded with a status of 504 (Outdated Optimize Dep)
```

### Your Environment

```text
Angular CLI: 20.3.8
Node: 24.1.0
Package Manager: yarn 4.9.2
OS: darwin arm64

Angular: 20.3.9
... animations, common, compiler, compiler-cli, core, forms
... language-service, localize, platform-browser
... platform-browser-dynamic, router

Package Version
---------------------------------------
@angular-devkit/architect 0.2003.8
@angular-devkit/build-angular 20.3.8
@angular-devkit/core 20.3.8
@angular-devkit/schematics 20.3.8
@angular/cdk 20.2.11
@angular/cli 20.3.8
@angular/material 20.2.11
@schematics/angular 20.3.8
rxjs 7.8.2
typescript 5.8.3
zone.js 0.15.1
```

### Anything else relevant?

_No response_

Start with the package.json start:both reproduction and inspect the shared .angular/cache/.../vite optimizer directory while running both ng serve processes. The fix is complete when parallel servers for different configurations load successfully without 504 Outdated Optimize Dep errors.

Written by the indexing model from the issue text.