F5 silently falls back to a native folder picker when launch.json inputFolder is invalid

[chiaramooney]

Problem

When launch.json's inputFolder is missing, empty, or wrong (e.g. stale or wrong-arch output), pressing F5 silently pops the native "Select input folder to package" dialog mid-debug, with no explanation of why a file dialog appeared during debugging. A new user has no idea what it's asking for or why (see also #1 — the picker also defaults to the wrong folder).

Proposed solution

• In resolveDebugConfiguration, validate inputFolder: if absent, derive it from the workspace (see This repo is missing a LICENSE file #1); if present but has no .exe, show "Build output not found at <path> — build the project?" with a Build button.
• Only fall back to a picker as a last resort, and title it clearly (e.g. "WinApp: choose the build output to debug") so it's obviously part of F5.

Additional context

Observed with microsoft-winappcli.winapp v0.2.1-prerelease.6, VS Code 1.127.0. Shares the "resolve build output from the workspace" engine proposed in #1, so fixing that unblocks this too. Low–medium cost.