fix: import with schema=false

[tstirrat15]

Fixes #509

Description

There's a couple of misbehaviors around zed import that we want to fix:

• The command fails if the validation file doesn't have a schema even if schema=false is passed
• The command overwrites the schema if a schema is provided and schema=false is passed

Neither of these is desirable. This PR adds tests for those.

Changes

Added a flag mustDefineSchema inside the decoder that the import command sets to false.

Testing

Included as unit tests.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.03%. Comparing base (80de266) to head (aa73a5d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #510   +/-   ##
=======================================
  Coverage   43.03%   43.03%           
=======================================
  Files          38       38           
  Lines        6098     6098           
=======================================
  Hits         2624     2624           
  Misses       3208     3208           
  Partials      266      266           

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[tstirrat15]

LGTM