fix: avoid PosixPath.endswith() in run_yang_model_checks_task

[SAY-5]

Fixes #10818.

populate_yang_model_dirs was calling .endswith("/") on moddir, which is a pathlib.Path (not a str), causing the daily run_yang_model_checks_task to fail with AttributeError("'PosixPath' object has no attribute 'endswith'"). Pass an f"{moddir}/" source directly to rsync so the trailing slash is always present and drop the broken if block.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.49%. Comparing base (e502e1e) to head (38fab67).
⚠️ Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
ietf/submit/utils.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #10819   +/-   ##
=======================================
  Coverage   88.48%   88.49%           
=======================================
  Files         332      332           
  Lines       44838    44836    -2     
=======================================
  Hits        39676    39676           
+ Misses       5162     5160    -2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.