Ensure AutoPublishCodeSha256 resolves to a string

[marekaiv]

Issue #, if available:
n/a

Description of changes:
Check that AutoPublishCodeSha256 resolves to a string and fail gracefully if it doesn't.

Description of how you validated changes:
Added a unit test to check the failure and ran existing unit tests to confirm there's no regression.

Checklist:

• Add/update tests using:
  • Correct values - existing tests cover this field
  • Bad/wrong values (None, empty, wrong type, length, etc.)
  • Intrinsic Functions
• make pr passes
• Update documentation - not needed
• Verify transformed template deploys and application functions as expected

[codecov-commenter]

Codecov Report

Merging #2173 (1e710a5) into develop (e7a1496) will increase coverage by 0.70%.
The diff coverage is 97.64%.

@@             Coverage Diff             @@
##           develop    #2173      +/-   ##
===========================================
+ Coverage    93.58%   94.29%   +0.70%     
===========================================
  Files           90       95       +5     
  Lines         6124     6430     +306     
  Branches      1260     1291      +31     
===========================================
+ Hits          5731     6063     +332     
+ Misses         183      169      -14     
+ Partials       210      198      -12     

Impacted Files	Coverage Δ
samtranslator/model/lambda_.py	93.10% <ø> (ø)
samtranslator/plugins/globals/globals.py	99.05% <ø> (ø)
samtranslator/translator/logical_id_generator.py	100.00% <ø> (+9.09%)	⬆️
samtranslator/region_configuration.py	77.77% <63.63%> (-22.23%)	⬇️
samtranslator/translator/translator.py	97.26% <90.00%> (+0.07%)	⬆️
samtranslator/swagger/swagger.py	93.65% <91.89%> (+0.28%)	⬆️
samtranslator/model/sam_resources.py	92.19% <96.66%> (+0.95%)	⬆️
samtranslator/__init__.py	100.00% <100.00%> (ø)
samtranslator/feature_toggle/dialup.py	100.00% <100.00%> (ø)
samtranslator/feature_toggle/feature_toggle.py	100.00% <100.00%> (+12.16%)	⬆️
... and 26 more

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[hawflau]

It seems we support intrinsic Fn::Ref but not Fn::Sub, as shown in the test added.
If I understand correctly, Fn::Sub also resolves to a string. My question is - do we also want to support support Fn::Sub?

[marekaiv]

We do want to support Fn::Sub but would prefer to deal with that when we tackle intrinsics support more broadly.

[jfuss]

Any reason we didn't add unit tests? :)