Filter out b64-encoded data before sending to LLM

[ml-evs]

Closes #869

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 68.12%. Comparing base (42b5c71) to head (5c6a401).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
pydatalab/pydatalab/apps/chat/blocks.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #870   +/-   ##
=======================================
  Coverage   68.12%   68.12%           
=======================================
  Files          62       62           
  Lines        3890     3890           
=======================================
  Hits         2650     2650           
  Misses       1240     1240           

Files with missing lines	Coverage Δ
pydatalab/pydatalab/apps/chat/blocks.py	20.16% <0.00%> (ø)

[cypress]

datalab    Run #2361

Run Properties:   Passed #2361  •  17898cf752 ℹ️: Merge 5c6a401a7d94f723646793ee836fc88e89dae7df into 3d730785129775f2ff6ca6ca24a7...

Project	datalab
Branch Review	fix-chatblock
Run status	Passed #2361
Run duration	04m 39s
Commit	17898cf752 ℹ️: Merge 5c6a401a7d94f723646793ee836fc88e89dae7df into 3d730785129775f2ff6ca6ca24a7...
Committer	Matthew Evans
View all properties for this run ↗︎

---

Test results
Failures	0
Flaky	0
Pending	0
Skipped	0
Passing	132
View all changes introduced in this branch ↗︎