Measure branch coverage

AGENTS.md asks for full line and branch coverage of new code, but
coverage only measured lines, so codecov could not see an untaken
branch. Enable branch measurement for local runs and CI.

Partial branches now count against the project total, which drops it
below the old 95% target without any code getting less tested. New
code is held to 100% by the patch status, so make the project status
informational instead of moving the target.

Author: Pierre-Sassoulas

codecov.yml

@@ -5,6 +5,8 @@ coverage:
         target: 100%
     project:
       default:
-        target: 95%
+        # Reported but never blocking: new code is held to 100% by the patch
+        # status, and the total moves when the way it is measured changes.
+        informational: true
 comment:
   layout: "reach, diff, flags, files"

pyproject.toml

@@ -296,6 +296,7 @@ ini_options.filterwarnings = [
 ]
 
 [tool.coverage]
+run.branch = true
 paths.source = [ "pylint" ]
 report.include = [ "pylint/*" ]
 report.omit = [ "*/test/*" ]