Fix b-bit MinHash estimator correction

[dipeshbabu]

What

Correct the b-bit MinHash A(r, b) denominator to use the required 2^b exponent and add a focused regression test.

Why

The previous 2*b exponent produced a biased correction whenever b was greater than two and r was nonzero.

Impact

Jaccard estimates using a nonzero universe ratio now follow the published estimator. The r=0 behavior is unchanged.

Root cause

The exponentiation term was transcribed as multiplication.

Checks

• uv run pytest -q test/test_minhash.py (18 passed)
• uv run ruff check datasketch/b_bit_minhash.py test/test_minhash.py
• git diff --check

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Additional details and impacted files

@@            Coverage Diff            @@
##             master     #325   +/-   ##
=========================================
  Coverage          ?   69.71%           
=========================================
  Files             ?       21           
  Lines             ?     2787           
  Branches          ?        0           
=========================================
  Hits              ?     1943           
  Misses            ?      844           
  Partials          ?        0           

Flag	Coverage Δ
unittests	69.71% <100.00%> (?)

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[dipeshbabu]

@ekzhu could you review the PR?