Improve handling of errors from Anthropic

[anth-volk]

Fixes #2223.

This PR simplifies our approach to interacting with Claude, improves error handling, allows Claude to determine streaming chunk size (potentially making streaming faster), and improves code quality.

Following merging of this, accompanying changes to front end are also required. However, these API changes are not breaking, meaning they can be handled separately.

#2223 was caused by an "overloaded" error from Anthropic. Currently, we handle this by just indicating an error, but in this case, it's very much not our fault. This code (as well as the accompanying front-end changes) aims to highlight that the cause is, in fact, Claude.

[codecov]

Codecov Report

Attention: Patch coverage is 97.77778% with 2 lines in your changes missing coverage. Please review.

Project coverage is 75.84%. Comparing base (c347b6e) to head (6c92457).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
policyengine_api/services/ai_analysis_service.py	95.23%	0 Missing and 1 partial ⚠️
tests/unit/services/test_ai_analysis_service.py	96.77%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2225      +/-   ##
==========================================
+ Coverage   75.39%   75.84%   +0.44%     
==========================================
  Files          81       83       +2     
  Lines        3130     3192      +62     
  Branches      344      348       +4     
==========================================
+ Hits         2360     2421      +61     
  Misses        702      702              
- Partials       68       69       +1     

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