parse: apkindex: fix provider_priority never actually being an int

provider_priority was actually a string all along! make it an int as type hinting suggests, and add some tests for apkindex parsing.

cc: @Newbyte

pmb/parse/apkindex.py

@@ -93,10 +93,9 @@ def parse_next_block(path: Path, lines: list[str]) -> ApkindexBlock | None:
             ret[key] = []
     provider_priority = ret.get("provider_priority")
     if provider_priority:
-        try:
-            provider_priority = int(provider_priority)
-        except ValueError:
-            provider_priority = None
+        if not provider_priority.isdigit():
+            raise RuntimeError(f"Invalid provider_priority: '{provider_priority}' parsing block {ret}")
+        provider_priority = int(provider_priority)
     else:
         provider_priority = None