feat(ai): add single message truncation

[shellmayr]

• Realized in this discussion that we don't handle the case of the single large message well at the moment

Truncate the message list to

• the last message, with its content truncated by characters, if the last message's serialized size exceeds a byte threshold; otherwise,
• the maximum number of messages, starting from the end of the list, whose total serialized size does not exceed a byte threshold.

Contributes to TET-1208

[linear]

TET-1208 Python SDK: truncate gen_ai messages

[alexander-alderman-webb]

Looks good in terms of the general approach.

The message mutation also to be addressed; we should not edit the return value of another library and return the edited value to the user.

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.92%. Comparing base (659bd84) to head (768a5b7).
⚠️ Report is 7 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
sentry_sdk/ai/utils.py	88.88%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5079   +/-   ##
=======================================
  Coverage   83.92%   83.92%           
=======================================
  Files         179      179           
  Lines       17948    17967   +19     
  Branches     3193     3198    +5     
=======================================
+ Hits        15062    15079   +17     
- Misses       1913     1914    +1     
- Partials      973      974    +1     

Files with missing lines	Coverage Δ
sentry_sdk/ai/utils.py	91.20% <88.88%> (-0.80%)	⬇️

... and 3 files with indirect coverage changes

[alexander-alderman-webb]

Removed the if result because result always has at least one element due to this PR.