fix: update pk field only raises unfriendly error

[waketzheng]

Description

Close #1870

Motivation and Context

Explicit raises OptionalError when executing await model.save(update_fields=['id'])

How Has This Been Tested?

make ci

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added the changelog accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codspeed-hq]

CodSpeed Performance Report

Merging #1873 will not alter performance

_{Comparing waketzheng:update-pk-field-only (4a1a39c) with develop (7cb24a5)}

Summary

✅ 12 untouched benchmarks

[coveralls]

Pull Request Test Coverage Report for Build 13179624319

Details

• 5 of 5 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.002%) to 89.236%

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Totals
Change from base Build 13160076672:	0.002%
Covered Lines:	6491
Relevant Lines:	7090

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💛 - Coveralls

[henadzit]

I would suggest to have a separate test case for this os it is documented with tests and it is not just a check in an existing test.

I would also suggest another test case:

o.save()
o.id = 123
with pytest.raises(OperationalError):
  o.save()

[waketzheng]

I tried, but it's hard to do that -- because can't get the original id if it changed which make it difficult to know whether object only changed pk field value.