Fix memory leak caused by not visiting the function in a CallValidator during gc

[dmontagu]

Fixes pydantic/pydantic#8239, I wouldn't be surprised if there was some connection to other memory issues we've noticed with ValidateCall (e.g., pydantic/pydantic#7687, though that one seemed to be addressed by doing a full garbage collection), either way this definitely seems like a bug.

@davidhewitt not sure if it makes sense to add tests of any sort for this, otherwise I think we can just merge.

[codecov]

Codecov Report

Merging #1100 (ee81c9f) into main (c7daf16) will increase coverage by 0.00%.
Report is 2 commits behind head on main.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1100   +/-   ##
=======================================
  Coverage   89.70%   89.70%           
=======================================
  Files         106      106           
  Lines       16359    16363    +4     
  Branches       35       35           
=======================================
+ Hits        14675    14679    +4     
  Misses       1677     1677           
  Partials        7        7           

Files	Coverage Δ
src/validators/call.rs	98.11% <ø> (ø)

... and 1 file with indirect coverage changes

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[codspeed-hq]

CodSpeed Performance Report

Merging #1100 will not alter performance

_{Comparing dmontagu/fix-validate-call-memory-leak (ee81c9f) with main (5d64894)}

Summary

✅ 140 untouched benchmarks

[davidhewitt]

I think for the moment it's ok to not test this for now; I think in principle we could build a fuzzer which builds schemas and checks that they get garbage collected, but it'd be a lot of work. Maybe we could build something reusable for the PyO3 ecosystem sometime.