Fix decoratornames for decorators without qname

[gaoflow]

Summary

• skip inferred decorator objects that do not expose a callable qname() in FunctionDef.decoratornames()
• add a regression test for @slice(0), which currently infers a Slice node and crashes while checking whether the function is a property

Fixes #3115.

Verification

• PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 python -m pytest tests/test_regrtest.py::NonRegressionTests::test_decorator_names_inference_error_leaking tests/test_regrtest.py::NonRegressionTests::test_decorator_names_skip_inferred_nodes_without_qname tests/test_regrtest.py -q
• PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 python -m pytest tests/test_scoped_nodes.py -q
• uv run --with ruff ruff check astroid/nodes/scoped_nodes/scoped_nodes.py tests/test_regrtest.py
• uv run --with black black --check astroid/nodes/scoped_nodes/scoped_nodes.py tests/test_regrtest.py
• python -m compileall -q astroid/nodes/scoped_nodes/scoped_nodes.py tests/test_regrtest.py
• git diff --check

I also tried a full local PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 python -m pytest -q; it reached 1979 passed but failed on local environment/optional dependency gaps (pytest-benchmark fixture missing, pkg_resources missing, numpy/pydantic brain expectations, and a symlink path test). The focused affected suites above pass.

[codspeed-hq]

Merging this PR will not alter performance

✅ 3 untouched benchmarks
⏩ 1 skipped benchmark¹

---

_{Comparing gaoflow:fix-decoratornames-slice-qname (45c8118) with main (65f1e61)}

Footnotes

1. 1 benchmark was skipped, so the baseline result was used instead. If it was deleted from the codebase, click here and archive it to remove it from the performance reports. ↩

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.59%. Comparing base (65f1e61) to head (45c8118).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3118   +/-   ##
=======================================
  Coverage   93.59%   93.59%           
=======================================
  Files          92       92           
  Lines       11371    11373    +2     
=======================================
+ Hits        10643    10645    +2     
  Misses        728      728           

Flag	Coverage Δ
linux	93.46% <100.00%> (+<0.01%)	⬆️
pypy	93.59% <100.00%> (+<0.01%)	⬆️
windows	93.57% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
astroid/nodes/scoped_nodes/scoped_nodes.py	93.29% <100.00%> (+0.01%)	⬆️

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[DanielNoord]

Please use isintance instead of getattr