Preserve sequence length with ambiguous elements

[Boulea7]

Type of Changes

	Type
✓	🐛 Bug fix

Description

Tuple and list concatenation expanded an element with multiple inferred values into multiple sequence items. This produced the wrong sequence length and caused downstream false positives when unpacking.

Each source element now contributes exactly one result. A unique inferred value stays precise, while an ambiguous or uninferable value becomes an unknown element.

Closes pylint-dev/pylint#2621

Verification

• Full test suite: 2078 passed, 66 skipped, 15 xfailed
• Inference tests: 462 passed, 7 skipped, 10 xfailed
• Ruff, Black, mypy, Pylint, and diff checks passed
• The downstream Pylint reproducer no longer reports unbalanced-tuple-unpacking

The complete pre-commit command could not initialize its copyright hook because git fetch could not connect to github.com on port 443. The corresponding local checks above passed.

[DanielNoord]

Can you rebase this and use towncrier to create the news fragment?

[codspeed-hq]

Merging this PR will not alter performance

✅ 3 untouched benchmarks
⏩ 1 skipped benchmark¹

---

_{Comparing Boulea7:fix/sequence-concat-ambiguous-elements (3739fd6) with main (04ba439)}

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 93.65%. Comparing base (04ba439) to head (3739fd6).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3221      +/-   ##
==========================================
- Coverage   93.65%   93.65%   -0.01%     
==========================================
  Files          93       93              
  Lines       11583    11581       -2     
==========================================
- Hits        10848    10846       -2     
  Misses        735      735              

Flag	Coverage Δ
linux	93.51% <100.00%> (-0.01%)	⬇️
pypy	93.65% <100.00%> (-0.01%)	⬇️
windows	93.62% <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/protocols.py	92.21% <100.00%> (-0.04%)	⬇️

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.