fix(package_info_plus): prefer compile-time build name and number on web

[Ortes]

Description

On the web, version / buildNumber come from version.json, fetched from the server at runtime. It describes the currently deployed bundle, so a client running a stale cached bundle reports the newer version.

Since Flutter 3.47.0 (flutter/flutter#187935), flutter_tools injects the app's build name and number as the dart-defines FLUTTER_BUILD_NAME / FLUTTER_BUILD_NUMBER (exposed as appBuildName / appBuildNumber). They are compiled into the running bundle.

This PR makes the web implementation use them for version / buildNumber when defined:

• read with bool.hasEnvironment, so the minimum Flutter version is unchanged; on older SDKs they're undefined and nothing changes;
• appName / packageName still come from version.json;
• the version stays correct even if the version.json fetch fails;
• native platforms are untouched.

The web plugin constructor gains two optional positional parameters (build name / number) for tests, defaulting to the compiled values. The mocked web tests pass null so they keep covering the version.json path. Two new tests cover the compile-time path.

Related Issues

• Fix [package_info_plus]: Use Flutter's compile-time build name/number on web instead of version.json #3989
• Fix [Bug]: [package_info_plus] Web: Version info returns next version in server #2675
• Related feat(package_info_plus): add opt-in compile-time package info accessor #3874 (earlier opt-in approach, closed once the Flutter side landed)

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I titled the PR using Conventional Commits.
• I did not modify the CHANGELOG.md nor the plugin version in pubspec.yaml files.
• All existing and new tests are passing. (web integration tests not run locally; relying on CI)
• The analyzer (flutter analyze) does not report any problems on my PR.

Breaking Change

• Yes, this is a breaking change.
• No, this is not a breaking change.