Validate format of event stream version CLI argument when passed (#1231)

This adds validation of the user-specified `--event-stream-version` (or
`--experimental-event-stream-version`) command-line arguments when they
are passed, to ensure they have a valid format.

### Motivation:

Currently, these flags only accept integers like `0`, `1`, etc. But I am
moving towards making the entry point and event stream versions align
with the Swift version, rather than having an independent versioning
scheme which users must look up or keep track of. Once this effort is
finished, in order to use the event stream format included in (for
example) Swift 6.3, a user would pass `--event-stream-version 6.3`.

In swiftlang/swift-package-manager#8944, I
recently landed a SwiftPM change which will permit any arbitrary string
for these arguments, and this means we need to begin validating the
format of supported versions within the testing library. In a subsequent
PR, I plan to introduce support for non-integer versions, at which point
having some existing validation logic will be even more valuable.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: stmontgomery

Sources/Testing/ABI/EntryPoints/EntryPoint.swift

@@ -255,15 +255,16 @@ public struct __CommandLineArguments_v0: Sendable {
   /// whichever occurs first.
   public var eventStreamOutputPath: String?
 
-  /// The version of the event stream schema to use when writing events to
-  /// ``eventStreamOutput``.
+  /// The value of the `--event-stream-version` or `--experimental-event-stream-version`
+  /// argument, representing the version of the event stream schema to use when
+  /// writing events to ``eventStreamOutput``.
   ///
   /// The corresponding stable schema is used to encode events to the event
   /// stream. ``ABI/Record`` is used if the value of this property is `0` or
   /// higher.
   ///
   /// If the value of this property is `nil`, the testing library assumes that
-  /// the newest available schema should be used.
+  /// the current supported (non-experimental) version should be used.
   public var eventStreamVersion: Int?
 
   /// The value(s) of the `--filter` argument.
@@ -376,16 +377,23 @@ func parseCommandLineArguments(from args: [String]) throws -> __CommandLineArgum
       }
     }
     if let eventOutputVersionIndex, !isLastArgument(at: eventOutputVersionIndex) {
-      result.eventStreamVersion = Int(args[args.index(after: eventOutputVersionIndex)])
+      let versionString = args[args.index(after: eventOutputVersionIndex)]
+
+      // If the caller specified a version that could not be parsed, treat it as
+      // an invalid argument.
+      guard let eventStreamVersion = Int(versionString) else {
+        let argument = allowExperimental ? "--experimental-event-stream-version" : "--event-stream-version"
+        throw _EntryPointError.invalidArgument(argument, value: versionString)
+      }
 
       // If the caller specified an experimental ABI version, they must
       // explicitly use --experimental-event-stream-version, otherwise it's
       // treated as unsupported.
-      if let eventStreamVersion = result.eventStreamVersion,
-         eventStreamVersion > ABI.CurrentVersion.versionNumber,
-         !allowExperimental {
+      if eventStreamVersion > ABI.CurrentVersion.versionNumber, !allowExperimental {
         throw _EntryPointError.experimentalABIVersion(eventStreamVersion)
       }
+
+      result.eventStreamVersion = eventStreamVersion
     }
   }
 #endif

Tests/TestingTests/SwiftPMTests.swift

@@ -308,6 +308,13 @@ struct SwiftPMTests {
       _ = try configurationForEntryPoint(withArguments: ["PATH", "--event-stream-version", "\(experimentalVersion)"])
     }
   }
+
+  @Test("Invalid event stream version throws an invalid argument error")
+  func invalidEventStreamVersionThrows() {
+    #expect(throws: (any Error).self) {
+      _ = try configurationForEntryPoint(withArguments: ["PATH", "--event-stream-version", "xyz-invalid"])
+    }
+  }
 #endif
 #endif