test: fix FakePluginAuthError LocalizedError conformance on the CI toolchain

[datlechin]

Problem

main's macOS App Tests job has been failing to compile since #1846. The test helper does not build on CI's toolchain (Xcode 26.4.1):

error: type 'FakePluginAuthError' does not conform to protocol 'LocalizedError'
private struct FakePluginAuthError: PluginDriverError {

PluginDriverError refines LocalizedError and supplies a default errorDescription in an extension inside TableProPluginKit, which is built with Library Evolution (BUILD_LIBRARY_FOR_DISTRIBUTION = YES). On the Swift compiler in Xcode 26.4.1, that inherited-requirement default is not picked up as the witness for a type conforming in a different module (the test target), so FakePluginAuthError is treated as not conforming to LocalizedError. The same code builds on Xcode 27, which is why it was not caught locally.

Fix

Give FakePluginAuthError an explicit errorDescription, so its LocalizedError conformance no longer depends on the cross-module default being witnessed. Test-only change, no TableProPluginKit ABI impact.

Verification

The test target compiles again on Xcode 26.4.1. TableProTests is not in the SwiftLint included scope, so the file's existing lint state is unchanged. No CHANGELOG entry: internal test build fix, not user-facing.