[google_font] Move the httpClient from a package level variable, to be on the Config object

[bramp]

In progress: I have not written changelog/documentation updates. I want to get early feedback / validation on this idea before I do the remaining work.

Move the httpClient from a package level variable, to be on the Config object

This allows the user to customise the httpClient as well as mock it out in tests.

Fixes #182429

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[gemini-code-assist]

Code Review

This pull request refactors the google_fonts package to move the httpClient from a package-level variable into the GoogleFonts.config object. This is a positive change that improves the package's testability and allows users to provide a custom http.Client. The changes are applied consistently across the library and its tests. My feedback focuses on preventing a potential resource leak with the new httpClient implementation.

[gemini-code-assist]

This is a great change for testability and customization! However, the current implementation creates an http.Client that is never closed, which can lead to resource leaks.

A better approach is to make httpClient nullable. This clarifies ownership: if a user provides a client, they are responsible for closing it. If it's null, the library can manage the client's lifecycle.

Here's a suggested implementation for the Config class:

  /// The HTTP client used to fetch fonts.
  ///
  /// If this is null, a new [http.Client] will be created for each request.
  /// If you are making many font requests, it is recommended to provide your
  /// own long-lived [http.Client].
  ///
  /// If you supply a client, you are responsible for closing it.
  http.Client? httpClient;

Then, in _httpFetchFontAndSaveToDevice in google_fonts_base.dart, you would handle the nullable client like this to ensure it's closed if created on-the-fly:

// in _httpFetchFontAndSaveToDevice
final client = GoogleFonts.config.httpClient ?? http.Client();
try {
  response = await client.get(uri);
} finally {
  if (GoogleFonts.config.httpClient == null) {
    // We created this client, so we should close it.
    client.close();
  }
}

  /// The HTTP client used to fetch fonts.
  ///
  /// If this is null, a new [http.Client] will be created for each request.
  /// If you are making many font requests, it is recommended to provide your
  /// own long-lived [http.Client].
  ///
  /// If you supply a client, you are responsible for closing it.
  http.Client? httpClient;