fix(server): build the xctest runner-error regex inline instead of a shared static

What changed

XCResultParser.isRunnerError keeps matching the synthetic xctest (<pid>) encountered an error name with the same /xctest \(\d+\) encountered an error/ regex, but builds it inline instead of caching it in a private static let.

Why

Regex<Substring> isn’t Sendable, so the shared static let xctestRunnerErrorRegex (added by #11606) tripped Swift 6 concurrency checking:

static property 'xctestRunnerErrorRegex' is not concurrency-safe because
non-'Sendable' type 'Regex<Substring>' may have shared mutable state

That failed the SwiftPM Build check on main (the package compiles in Swift 6 language mode). The problem was the shared static, not the regex itself — nonisolated(unsafe) would only silence the checker rather than remove the shared mutable global.

Building the regex inline in isRunnerError sidesteps the shared state entirely: no unsafe, nothing to prove Sendable, and the declarative pattern stays. isRunnerError runs per “Test Case” node, but the per-call cost is negligible next to xcresult parsing (xcresulttool subprocess + JSON decoding), which dominates.

Validation

• Compiles clean under -swift-version 6.
• Behavior is unchanged — it’s the same regex and wholeMatch, just not cached.

Once merged this restores the SwiftPM Build check on main.

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