Remove build_runner dependency and migrate client build to Dart script

[jyameo]

This change decouples dwds from package:build_runner and its related dependencies (build_web_compilers, build_daemon, build, build_version, and pubspec_parse) to prepare for migration into the Dart SDK workspace.

Key Changes:

• Removed build.yaml and copy_builder.dart. Introduced tool/build.dart which uses dart compile js for building the injected client and generates version files.
• Removed build_runner, build, build_daemon, build_web_compilers, build_version and pubspec_parse from dev_dependencies.
• Refactored ensure_version_test.dart and min_sdk_test.dart to use package:yaml.
• Added test/build/build_script_test.dart to validate the new build script.
• Moved all integration tests into dwds/test/integration. Note: Test fixtures and integration tests relying on build_daemon are temporarily broken and will be addressed post-migration.

Validations:

• Verified dart run tool/build.dart correctly generates client.js and version.dart.

Fixes dart-lang/sdk#62475