fix: accept numeric zero for RangeThreshold min/max (#681)

[sankalpsthakur]

Fixes #681

Proposed Changes

RangeThreshold.min / max (and greater/lesser value) are required floats. Validation must use identity checks against None so numeric zero (0 / 0.0) is a valid bound — not treated as “missing” the way a truthiness check (if not min) would.

This change:

• Documents that intent on the setters (is None only; zero and within=False stay valid).
• Adds regression tests for the issue reproduction (RangeThreshold(..., min=0, max=10, within=True)), serialization/create_check request bodies, and deserialize of min: 0.

threshold = RangeThreshold(level=CheckStatusLevel.OK, min=0, max=10, within=True)
# min stays 0; create_check JSON includes "min": 0

Checklist

• CHANGELOG.md updated
• Rebased/mergeable
• A test has been added if appropriate
• pytest tests/test_Thresholds.py::ThresholdZeroValueTest completes successfully
• Commit messages are conventional
• Sign CLA (if not already signed)

AI/LLM disclosure

• AI coding tools (including Grok and/or Codex agent-assisted editing) were used to help draft or modify code and this PR description.
• I reviewed the complete change, understand the reasoning, and ran the reported local tests before submitting.
• This submission is original work of authorship under the project CLA / contributor terms; AI output was not pasted unreviewed.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.41%. Comparing base (9ca599f) to head (31b0508).
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #715      +/-   ##
==========================================
+ Coverage   90.38%   90.41%   +0.02%     
==========================================
  Files          40       40              
  Lines        3546     3546              
==========================================
+ Hits         3205     3206       +1     
+ Misses        341      340       -1     

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