fix(backups): include pgBackRest error details in S3 stanza failure messages

[marceloneppel]

Issue

Solution

Extract and append error hints from pgBackRest stdout/stderr to blocked status messages during stanza initialization and checks, improving debuggability of S3-related failures. Use prefix matching for S3 block messages to accommodate the new error suffixes.

Checklist

• I have added or updated any relevant documentation.
• I have cleaned any remaining cloud resources from my accounts.

[codecov]

Codecov Report

❌ Patch coverage is 55.81395% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.54%. Comparing base (89a7e08) to head (1aaebc9).

Files with missing lines	Patch %	Lines
src/backups.py	53.65%	15 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1243      +/-   ##
==========================================
- Coverage   72.82%   72.54%   -0.28%     
==========================================
  Files          16       16              
  Lines        4117     4152      +35     
  Branches      602      610       +8     
==========================================
+ Hits         2998     3012      +14     
- Misses        893      910      +17     
- Partials      226      230       +4     

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