move ensure_read_supported from Scan to Snapshot

[zachschuermann]

previously, we were only checking protocol.ensure_read_supported() when we create a Scan, instead we should move it up earlier to Snapshot creation. this is useful so that we can fail earlier and generally as soon as we get a Protocol we should ensure read is supported instead of waiting for Scan creation. note this is useful for CDF too (as it relies on snapshots but separate scans)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.24%. Comparing base (5a133d3) to head (fe1735c).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #518      +/-   ##
==========================================
- Coverage   80.25%   80.24%   -0.01%     
==========================================
  Files          61       61              
  Lines       13403    13402       -1     
  Branches    13403    13402       -1     
==========================================
- Hits        10756    10755       -1     
  Misses       2093     2093              
  Partials      554      554              

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[scovich]

Definitely a narrower waist, thanks.