fix(langchain): handle case when parent span wasn't traced

[rbasoalto]

It's possible for the parent span to not have been traced (or have been GCd) so a KeyError would be raised when trying to fetch the span for the parent run_id. Now we defensively .get() the parent span instead of subscripting it.

[sentrivana]

Thank you @rbasoalto!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.31%. Comparing base (8a7e226) to head (a7bbaaf).
Report is 1 commits behind head on master.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3656      +/-   ##
==========================================
- Coverage   84.32%   84.31%   -0.01%     
==========================================
  Files         133      133              
  Lines       14063    14063              
  Branches     2379     2379              
==========================================
- Hits        11858    11857       -1     
+ Misses       1472     1471       -1     
- Partials      733      735       +2     

Files with missing lines	Coverage Δ
sentry_sdk/integrations/langchain.py	68.87% <100.00%> (ø)

... and 3 files with indirect coverage changes