find-updates: resolve via source_resolver when present #1401

Part of #1294 (Phase 8 of epic #1254). Branch: new-resolver-config.

What needs to happen

Update _find_newer_versions() in src/fromager/commands/find_updates.py: it currently calls overrides.find_and_invoke("get_resolver_provider", ...) directly, so packages with source: config silently resolve through the legacy resolver. Use sources.get_source_provider() when pbi.source_resolver is present; fall back to the legacy path otherwise.

This is the one piece of #1294 with a live behavioral gap today.

Tests

• find-updates for a new-config package resolves via the configured provider
• Legacy-config packages behave as before

Notes

Independent of the other #1294 sub-issues; can be worked on in any order.