test: Improve FileManager PathTooLong assert

[philipphofmann]

Previously, the test verified the number of logged messages, causing fragile failures if unrelated logs were added. Now it asserts the presence of the specific fatal log message, ensuring stability even when additional logs are introduced.

Test failed here: https://github.com/getsentry/sentry-cocoa/actions/runs/18740554352/job/53455824782?pr=6508

#skip-changelog

Closes #6511

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.939%. Comparing base (fe83264) to head (1a770fd).
⚠️ Report is 7 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #6510       +/-   ##
=============================================
- Coverage   86.996%   86.939%   -0.058%     
=============================================
  Files          452       451        -1     
  Lines        37691     37547      -144     
  Branches     17488     17388      -100     
=============================================
- Hits         32790     32643      -147     
+ Misses        4854      4628      -226     
- Partials        47       276      +229     

see 47 files with indirect coverage changes

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[philprime]

LGTM, thank you for pinning the assertion down to the actual message