[Bug] Fix regex for urls in image function

[kklemon]

The regex for matching urls in the image function starts with a $ character (ref), i.e. the "match end of string"-metacharacter, which seems to be wrong. I guess the intention was to match against the start of the string, which could be achieved with a ^ character, but since re.match matches against the start of a string already, this can also be omitted in this specific case.

This led to the situation that urls are generally not recognized and can't be used with the image function. This PR attempts to fix this issue by modifying the aforementioned regex and additionally providing tests for the _image module to prevent such problems in the future.

[riedgar-ms]

Tag @nking-1

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All modified and coverable lines are covered by tests ✅

Project coverage is 61.43%. Comparing base (4ae2ea6) to head (370c830).

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #928      +/-   ##
==========================================
+ Coverage   56.43%   61.43%   +4.99%     
==========================================
  Files          63       63              
  Lines        4791     4792       +1     
==========================================
+ Hits         2704     2944     +240     
+ Misses       2087     1848     -239     

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[nking-1]

This looks like a solid fix with tests. We'll probably be changing the internals of how images are stored and serialized in the model, but at a high level it shouldn't impact the fixes here. I think we can go ahead and take this change, and if any further work is needed I can do it as part of the upcoming multimodal support feature

[riedgar-ms]

@kklemon , I think you may be hitting this issue in the tests:
https://stackoverflow.com/questions/6416782/what-is-namedtemporaryfile-useful-for-on-windows
It may be simpler to create a temporary directory within the test fixture, and yield that. Then create the image as a regular file within that.