@angular/build:karma ignores overrides in karma.conf.js

### Command

test

### Is this a regression?

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

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

_No response_

### Description

Using the new `@angular/build:karma` builder, coverage overrides defined in the `karma.conf.js` are ignored.

### Minimal Reproduction

* clone https://github.com/fynnfeldpausch/karma-coverage
* take a look at `app.ts` - there is an additional method
* take a look at `karma.conf.js` and the defined coverage
* global coverage is set to 80%
* local coverage is set to 0% for `app.ts`
* run `ng test -- --no-watch --no-progress --browsers=ChromeHeadless --code-coverage`

### Exception or Error

```text
Test run fails with coverage errors:

19 08 2025 15:57:46.079:ERROR [coverage]: Chrome Headless 139.0.0.0 (Mac OS 10.15.7): Coverage for statements (75%) does not meet per-file (/repro-app/src/app/app.ts) threshold (80%)
19 08 2025 15:57:46.079:ERROR [coverage]: Chrome Headless 139.0.0.0 (Mac OS 10.15.7): Coverage for lines (66.66%) does not meet per-file (/repro-app/src/app/app.ts) threshold (80%)
19 08 2025 15:57:46.079:ERROR [coverage]: Chrome Headless 139.0.0.0 (Mac OS 10.15.7): Coverage for functions (0%) does not meet per-file (/repro-app/src/app/app.ts) threshold (80%)
```

### Your Environment

```text
_ _ ____ _ ___
/ \ _ __ __ _ _ _| | __ _ _ __ / ___| | |_ _|
/ △ \ | '_ \ / _` | | | | |/ _` | '__| | | | | | |
/ ___ \| | | | (_| | |_| | | (_| | | | |___| |___ | |
/_/ \_\_| |_|\__, |\__,_|_|\__,_|_| \____|_____|___|
|___/

Angular CLI: 20.1.6
Node: 24.2.0
Package Manager: npm 11.3.0
OS: darwin arm64

Angular: 20.1.7
... common, compiler, compiler-cli, core, forms
... platform-browser, router

Package Version
------------------------------------------------------
@angular-devkit/architect 0.2001.6
@angular-devkit/core 20.1.6
@angular-devkit/schematics 20.1.6
@angular/build 20.1.6
@angular/cli 20.1.6
@schematics/angular 20.1.6
rxjs 7.8.2
typescript 5.8.3
zone.js 0.15.1
```

### Anything else relevant?

_No response_

リンクされた karma-coverage の再現ケースをクローンし、app.ts と karma.conf.js を調査してから、記載された ng test コマンドを実行してカバレッジの失敗を再現します。@angular/build:karma が設定をどのように処理するかを追跡し、グローバルなしきい値が引き続き有効なまま、ファイルごとのカバレッジオーバーライドが適用されることを確認します。

索引モデルが issue の本文から書いたものです。