[METRICS SDK] Enforce a runtime minimum scale for Base2ExponentialHistogramAggregation

[ThomsonTan]

Fixes #4325

Changes

This change enforces a runtime minimum scale of -10 for base2 exponential histograms, guarantees sufficient bucket capacity at that floor to preserve recorded counts, centralizes scale reduction across record, merge, and diff paths, guards the indexer against oversized shifts, and adds regression tests plus changelog documentation for the new behavior.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

❌ Patch coverage is 92.37288% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.67%. Comparing base (754928d) to head (e51410f).

Files with missing lines	Patch %	Lines
...egation/base2_exponential_histogram_aggregation.cc	92.04%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4353      +/-   ##
==========================================
+ Coverage   86.66%   86.67%   +0.02%     
==========================================
  Files         525      525              
  Lines       20481    20543      +62     
==========================================
+ Hits        17748    17804      +56     
- Misses       2733     2739       +6     

Files with missing lines	Coverage Δ
...metry/sdk/metrics/aggregation/aggregation_config.h	100.00% <ø> (ø)
...aggregation/base2_exponential_histogram_indexer.cc	100.00% <100.00%> (ø)
...egation/base2_exponential_histogram_aggregation.cc	95.26% <92.04%> (-1.37%)	⬇️

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[lalitb]

Thanks.