[flutter_tools] Handle missing Xcode gracefully in getInfo and buildMacOS (#191180)

Fixes https://github.com/flutter/flutter/issues/191176

### Problem
In Flutter 3.47.0 telemetry, an unhandled `ProcessException` when
invoking `xcodebuild` emerged as a top crash signature. When running
`flutter build macos` on a machine where Xcode is not installed,
incomplete, or misconfigured, `flutter_tools` crashes with an unhandled
`ProcessException` instead of providing a graceful `ToolExit` error
message.

### Root Cause
1. `buildMacOS` in
`packages/flutter_tools/lib/src/macos/build_macos.dart` lacked
pre-flight toolchain validation
(`globals.xcodeProjectInterpreter?.isInstalled != true`) before querying
Xcode project information.
2. In `XcodeProjectInterpreter.getInfo`
(`packages/flutter_tools/lib/src/ios/xcodeproj.dart`), `xcodebuild
-list` is executed with `throwOnError: true` while only allowing exit
codes 66 and 74. When `xcrun` returns exit code 72 (unable to find
utility "xcodebuild"), `_processUtils.run` throws an uncaught
`ProcessException`.
3. `buildMacOS` improperly used non-null assertions on `projectInfo!`
when resolving build schemes.

### Fix
- Added pre-flight Xcode toolchain check in `buildMacOS` to exit early
with `globals.userMessages.xcodeMissing`.
- Wrapped `XcodeProjectInterpreter.getInfo` in a `try ... on
ProcessException` block to convert unexpected process failures to
user-friendly `ToolExit` messages.
- Added safe null checks on `projectInfo` in `buildMacOS`.