Add support for .expectationFailed in interop and exit tests.

[grynspan]

This PR enhances the JSON event stream to allow us to recognize and recover issues of kind .expectationFailed. This then improves the fidelity of these issues when passed through XCTest interop and exit test process boundaries.

For example, given the following exit test:

@Test func foo() async {
  await #expect(processExitsWith: .success) {
    struct S: Equatable {
      var x: Int
      var y: String
    }
    let lhs = S(x: 1, y: "abc")
    let rhs = S(x: 2, y: "def")
    #expect(lhs == rhs)
  }
}

We'll now get a breakdown of the failed expectation (note I recorded this example with --verbose so type info would be visible too):

<X>  Test foo() recorded an issue at [...]: Expectation failed: lhs == rhs
`->  lhs == rhs: Swift.Bool → false
`->    lhs: ModuleName.S → S(x: 1, y: "abc")
`->      x: Swift.Int → 1
`->      y: Swift.String → "abc"
`->    rhs: ModuleName.S → S(x: 2, y: "def")
`->      x: Swift.Int → 2
`->      y: Swift.String → "def"

In the process of implementing this change, I was able to reconcile the issue-decoding logic used in interop and exit tests so they share an implementation.

This PR also resolves an issue running interop tests inside Xcode 26.4 where the tests could spuriously fail.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[grynspan]

Not ready to make this public/SPI yet.

[grynspan]

Not ready to make these public/SPI yet.

[stmontgomery]

I know we separated the comment on withKnownIssue into a discrete field intentionally, so are we potentially conflating them by having them combined into a single field? (That might be a preexisting problem)

[grynspan]

As of right now, the known-issue comment isn't in the encoded form. It's something we can certainly look into as a separate PR.

[grynspan]

(The source comment itself is not new, but is out of date.)