Expose the app's build name and number as compile-time constants

[Ortes]

The tool already injects FLUTTER_VERSION, FLUTTER_CHANNEL, framework/engine revisions (exposed as FlutterVersion) and FLUTTER_APP_FLAVOR (exposed as appFlavor) as dart-defines. The application's own version: from pubspec.yaml is the one missing field, even though the tool already parses it (FlutterManifest.buildName/buildNumber) and embeds it into version.json, Info.plist and build.gradle on every build.

Without it, the running version is unknowable on the web: version.json is fetched from the server at runtime, so a stale cached client reports the freshly deployed version instead of its own, and version gates / update prompts built on package_info_plus silently fail for exactly the users they exist for (fluttercommunity/plus_plugins#2675).

This PR:

• injects FLUTTER_BUILD_NAME and FLUTTER_BUILD_NUMBER (from --build-name/--build-number, falling back to the pubspec version) in FlutterCommand.getBuildInfo, following the FLUTTER_APP_FLAVOR pattern including its reserved-name guards;
• exposes them as appBuildName / appBuildNumber constants in package:flutter/services.dart (new src/services/app_version.dart, same style as flavor.dart / flutter_version.dart);
• adds tool tests (injection from pubspec, no injection without a version, environment/dart-define collision guards) and framework tests.

Like all String.fromEnvironment constants, these are tree-shaken when unreferenced, so nothing ships in the binary unless the app reads them.

package_info_plus is ready to consume these defines (fluttercommunity/plus_plugins#3874), which would make the fix transparent for the ecosystem. A companion proposal for the pure-Dart side (dart compile, servers/CLIs) is dart-lang/sdk#38855 — neither subsumes the other, since Flutter does not build through dartdev.

Fixes #187934

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

[gemini-code-assist]

Code Review

This pull request introduces 'appBuildName' and 'appBuildNumber' constants to the services library, allowing applications to access their build name and build number at runtime. These values are injected into 'dartDefines' during the build process, with validation to prevent users from manually overriding them. Feedback suggests refining the validation logic when checking existing 'dartDefines' to avoid false positives on custom variables that share a prefix with the framework-defined build variables.