--output pointing at a nonexistent directory fails late with a misleading error #446

Repro

Run the CLI with any valid bundle and an --output path whose directory does not exist:

xchtmlreport SomeValid.xcresult --output /some/missing/dir/report

The tool does all the parsing work, then fails at the very end when writing index.html:

An error has occured while creating the report
Error: The file "index.html" doesn't exist.

Exit code is 1. Neither line mentions the actual problem — that the output directory doesn't exist.

Details

• Reproduces on Xcode 26.2 (local) and Xcode 26.6 (CI runners) — environment-independent.
• The failure point is the html.write(toFile:) call in Sources/XCTestHTMLReport/XCTestHtmlReport.swift (run()): the thrown CocoaError surfaces as ArgumentParser's misleading "The file "index.html" doesn't exist." message, after all parsing/rendering work is already done.
• This caused a false alarm in the first scheduled toolchain-drift run (Toolchain drift detected (Xcode 26.6 (17F113)) #444): the workflow passed an uncreated $RUNNER_TEMP subdirectory and the late, misleading error looked like toolchain drift.

Suggested fix (in the CLI)

Either create the output directory (FileManager.createDirectory(atPath:withIntermediateDirectories:)) before writing, or validate it up front in validate() and fail fast with a clear message ("output directory /some/missing/dir does not exist"). Failing late after minutes of parsing, with an error that points at the wrong file, is the bug regardless of which option is chosen.

The workflow-side workaround (mkdir before invoking) landed separately in #445; this issue tracks the product fix.