Fix typing in MJPG stream and fix system test

[DominicOram]

Fixes #776

Instructions to reviewer on how to test:

1. Run pyright . and confirm there's no errors
2. Comment out the skip on test_grid_overlay
3. Run this test and confirm that snapshot images are written to the top folder

Checks for reviewer

• Would the PR title make sense to a scientist on a set of release notes
• If a new device has been added does it follow the standards
• If changing the API for a pre-existing device, ensure that any beamlines using this device have updated their Bluesky plans accordingly
• Have the connection tests for the relevant beamline(s) been run via dodal connect ${BEAMLINE}

[DiamondJoseph]

Hyperion#183

Opportunity to prioritise #716 and continue moving off ophyd?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.55%. Comparing base (4130b0f) to head (31aba2a).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #777   +/-   ##
=======================================
  Coverage   94.55%   94.55%           
=======================================
  Files         115      115           
  Lines        4630     4631    +1     
=======================================
+ Hits         4378     4379    +1     
  Misses        252      252           

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