Don't stop discovery early when verification is enabled

When verify_content=True, we need fallback sources in case the first
OA source fails verification (e.g., PMC has wrong PDF associated with
a PMCID). Now discovery continues to find all sources (like Unpaywall)
so they can be tried when PMC fails.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: hherb

src/bmlibrarian/discovery/full_text_finder.py

@@ -224,9 +224,13 @@ def discover_and_download(
         if progress_callback:
             progress_callback("discovery", "starting")
 
+        # When verification is enabled, don't stop on first OA - we need fallback sources
+        # in case the first source fails verification (e.g., PMC has wrong PDF)
+        stop_early = self.prefer_open_access and not verify_content
+
         discovery = self.discover(
             identifiers,
-            stop_on_first_oa=self.prefer_open_access,
+            stop_on_first_oa=stop_early,
             progress_callback=progress_callback
         )