fix: validate files in FilesSnapshot before processing

[faizanakram99]

Add check to ensure each file is valid before processing. Without this change filemtime throws an error if the file doesn't exist but is part of cache, which is very common in dev environment.

For production it won't matter since is_file is cached, for some reason is_file is also faster than file_exists. Symfony codebase also prefers is_file over file_exists when checking existence of php file.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 94.79%. Comparing base (53f9d49) to head (79d0b1b).
⚠️ Report is 126 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Cache/FilesSnapshot.php	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #772      +/-   ##
============================================
- Coverage     95.72%   94.79%   -0.94%     
- Complexity     1773     1847      +74     
============================================
  Files           154      175      +21     
  Lines          4586     4880     +294     
============================================
+ Hits           4390     4626     +236     
- Misses          196      254      +58     

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