During a backup, LogExceptionHelper.LogCommonWarning is the shared helper used
to log "excluding path" warnings from many enumeration/metadata steps. For known,
expected problems (permission denied, locked file, path not found, path too long)
it passed the exception to WriteWarningMessage, so the file log, console and
report modules rendered the full stack trace (via LogEntry.AsString(true)) for
every such path — bloating the logs with no diagnostic value.
The known-category branches now log without attaching the exception, so no stack
trace is stored; the self-describing warning message (e.g. "Excluding path due to
permission denied: <path>") remains. The fallback branch keeps the exception so
genuinely unexpected failures are still fully debuggable.
Adds Issue6426 tests asserting known-problem warnings carry no exception while
unknown errors keep it.
Fixes#6426
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
