fix: decode non-utf-8 blob content

[jennifer-richards]

Reuses the algorithm that has long been applied to document contents.

[codecov]

Codecov Report

❌ Patch coverage is 93.54839% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.43%. Comparing base (15b2abd) to head (a2a32eb).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
ietf/doc/storage_utils.py	88.88%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10729      +/-   ##
==========================================
+ Coverage   88.42%   88.43%   +0.01%     
==========================================
  Files         331      331              
  Lines       44557    44606      +49     
==========================================
+ Hits        39399    39449      +50     
+ Misses       5158     5157       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.