Feat: add default-variable fallback when override-resolved artifact URL fails

[CodeWithKyrian]

This PR updates artifact URL resolution to retry with package default variables when the override-resolved URL is invalid, unresolved, or unavailable. If the default-based URL resolves successfully, Composer installs from that URL; if it also fails, behavior falls back to the existing Composer dist/source path.

Motivation and Context

Consumers can provide override variables that resolve to a non-existent artifact variant even when the package default variant exists and is valid for the same platform. Previously, that scenario could drop directly to base Composer fallback behavior. This change improves resilience by attempting a sensible in-plugin fallback first (defaults + {version}), so users are more likely to get a usable platform artifact without manual intervention.

What's Changed

• Updated installer URL resolution flow to attempt primary candidate using defaults + app overrides + {version}.
• Added retry candidate using defaults only + {version} when an override-based resolution fails.
• Kept final fallback behavior unchanged when both candidates fail.
• Added explicit logging for override failure and default-variable fallback usage.
• Updated README placeholder behavior section to document the new retry flow.

Breaking Changes

None.