Reject macOS balloon targets below the restore-image minimum

For macOS guests, "tart run --balloon-target-memory" now also enforces
the VM's own minimum supported memory size (dictated by the restore
image), similarly to how "tart set --memory" restricts the configured
memory size.

Addresses a PR review comment.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Sources/tart/Commands/Run.swift

@@ -429,7 +429,13 @@ struct Run: AsyncParsableCommand {
         + " configured memory size of \(vmConfig.memorySize / 1024 / 1024) MB")
     }
 
-    let minimumAllowedMemorySize = VZVirtualMachineConfiguration.minimumAllowedMemorySize
+    var minimumAllowedMemorySize = VZVirtualMachineConfiguration.minimumAllowedMemorySize
+    if vmConfig.os == .darwin {
+      // macOS guests additionally have a minimum supported memory size
+      // dictated by the restore image they were created from, similarly
+      // to how "tart set --memory" restricts the configured memory size
+      minimumAllowedMemorySize = max(minimumAllowedMemorySize, vmConfig.memorySizeMin)
+    }
     if targetMemoryBytes < minimumAllowedMemorySize {
       throw ValidationError("--balloon-target-memory (\(targetMemoryMB) MB) is too small,"
         + " it should be at least \(minimumAllowedMemorySize / 1024 / 1024) MB")

Tests/TartTests/MemoryBalloonTests.swift

@@ -78,6 +78,25 @@ final class MemoryBalloonTests: XCTestCase {
     XCTAssertNoThrow(try Run.validateBalloonTargetMemory(4096, vmConfig: vmConfig))
   }
 
+  func testBalloonTargetMemoryValidationRespectsDarwinMinimum() throws {
+    // A macOS guest whose restore image requires 4096 MB of memory at minimum
+    var vmConfig = VMConfig(platform: Linux(), cpuCountMin: 1, memorySizeMin: 4096 * 1024 * 1024)
+    vmConfig.os = .darwin
+    vmConfig.memoryBalloon = true
+    try vmConfig.setMemory(memorySize: 8192 * 1024 * 1024)
+
+    // Target below the restore image's minimum supported memory size
+    XCTAssertThrowsError(try Run.validateBalloonTargetMemory(2048, vmConfig: vmConfig))
+
+    // Target at the restore image's minimum supported memory size
+    XCTAssertNoThrow(try Run.validateBalloonTargetMemory(4096, vmConfig: vmConfig))
+
+    // The same minimum doesn't apply to Linux guests, similarly
+    // to how "tart set --memory" doesn't restrict them
+    vmConfig.os = .linux
+    XCTAssertNoThrow(try Run.validateBalloonTargetMemory(2048, vmConfig: vmConfig))
+  }
+
   func testBalloonDeviceOnlyConfiguredWhenEnabled() throws {
     // Disabled by default
     XCTAssertEqual(try craftConfiguration().memoryBalloonDevices.count, 0)