feat(sqlalchemy): maxLength validator from char_length constraints

[aiolibsbot]

What

Translate char_length(col) <= N / < N check constraints into maxLength input validators.

Why

_get_validators already turned char_length(col) >= N / > N constraints into minLength validators, and it handles the full ge/gt/le/lt range for numeric bounds. But the <= / < char-length cases fell through silently — a CheckConstraint(func.char_length(col) <= N) on a Text column (no fixed .length) produced zero client-side max-length validation. This closes that asymmetry. Part of #666.

How

Added le -> maxLength(N) and lt -> maxLength(N-1) branches alongside the existing ge/gt minLength handling. The -1 for the strict < mirrors the numeric lt -> maxValue(N-1) convention. Frontend already deserializes the maxLength validator, so no admin-js change.

Testing

Extended test_check_constraints with <= and < char-length columns asserting maxLength(5) and maxLength(4). Full suite: 97 passed, 1 xfailed; the lone test_admin_view failure is pre-existing (#934) and unrelated.

---

Quality Report

Changes: 2 files changed, 10 insertions(+)

Code scan: clean

Tests: failed (command not found)

Branch hygiene: clean

Generated by Kōan

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.64%. Comparing base (2448fd1) to head (fb91940).
⚠️ Report is 168 commits behind head on master.

Files with missing lines	Patch %	Lines
aiohttp_admin/backends/sqlalchemy.py	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1170      +/-   ##
==========================================
+ Coverage   95.56%   95.64%   +0.07%     
==========================================
  Files          21       21              
  Lines        2954     2962       +8     
  Branches      193      195       +2     
==========================================
+ Hits         2823     2833      +10     
+ Misses        105      103       -2     
  Partials       26       26              

Flag	Coverage Δ
integration	81.00% <0.00%> (+0.05%)	⬆️
js	83.89% <ø> (ø)
unit	97.51% <87.50%> (+0.09%)	⬆️

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