fix: add bounds checking to proxy tier prediction

[vdusek]

Summary

• Add bounds clamping in _ProxyTierTracker.predict_tier() to prevent negative or out-of-bounds tier index that could cause IndexError on proxy selection
• Normalize stored tier on entry and clamp after adjustment, using a cached max_tier local

Test plan

• Added test_predict_tier_bounds_with_single_tier to verify tier stays at 0 across repeated calls with a single-tier config
• All existing proxy tier tests pass

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.24%. Comparing base (5b28f03) to head (a3890a6).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1824      +/-   ##
==========================================
- Coverage   92.25%   92.24%   -0.01%     
==========================================
  Files         157      157              
  Lines       10886    10889       +3     
==========================================
+ Hits        10043    10045       +2     
- Misses        843      844       +1     

Flag	Coverage Δ
unit	92.24% <100.00%> (-0.01%)	⬇️

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[vdusek]

not a bug