@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_

Clone bản tái hiện karma-coverage được liên kết và kiểm tra app.ts cùng karma.conf.js, sau đó chạy lệnh ng test được liệt kê để tái hiện lỗi coverage. Theo dõi cách @angular/build:karma xử lý cấu hình và xác minh rằng tùy chỉnh ghi đè coverage theo từng tệp được tôn trọng, trong khi ngưỡng toàn cục vẫn có hiệu lực.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.