fix(test-forks): report tests dropped by validity markers and by generation

[mkzung]

Description

A filter reports what it deselects and a validity marker does not, so two ways of dropping a test look different in the same run. pytest_collection_modifyitems builds filter_stats and prints a line per function for --fill-until and friends, while a param dropped because its fork is outside the marker's validity range leaves nothing behind.

Generation has the same gap one step earlier. When test_fork_set or the intersection with the selected forks comes out empty, pytest_generate_tests drops the test without parametrizing it at all, so it never reaches collection and no count anywhere accounts for it. Only -vv shows it, as a skip.

Two commits, one for each gap. One records validity-marker deselections beside the existing filter stats. Its pair stashes the node id and the reason whenever generation is about to drop a test unparametrized, then reports the total, so a run says how many tests it never generated rather than silently omitting them.

Three tests cover it: a param-level deselection where every case of a function goes, an unparametrized count, and that reporting line itself. Reverting forks.py alone fails all three.

Related Issues or PRs

Follows #3461, which fixed the config that could not collect these directories at all.

Checklist

• Ran fast static checks to avoid CI fails, see Code Standards & Verifying Changes: just static
• PR title has the form <type>(<area>): <title>, where <type> and <area> come from an appropriate C-<type>, respectively A-<area>, label. The title should match the target squash commit message.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.90%. Comparing base (c4deda5) to head (4daa2ec).
⚠️ Report is 1 commits behind head on forks/amsterdam.

Additional details and impacted files

@@               Coverage Diff                @@
##           forks/amsterdam    #3468   +/-   ##
================================================
  Coverage            93.90%   93.90%           
================================================
  Files                  624      624           
  Lines                36905    36905           
  Branches              3326     3326           
================================================
  Hits                 34655    34655           
  Misses                1569     1569           
  Partials               681      681           

Flag	Coverage Δ
unittests	93.90% <ø> (ø)

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