Improve discriminated union error message for invalid union variants

[Viicos]

Remove duplicated test and improve test_plain_function_schema_is_invalid
to make use of a real model definition.

Change Summary

Closes #11158.

Note that more cleanups could be applied to the discriminator inference logic. I was able to remove one useless condition check, but we still raise the same error in two different places and with a bit of refactoring we can surely simplify it further.

Related issue number

Checklist

• The pull request title is a good summary of the changes - it will be used in the changelog
• Unit tests for the changes exist
• Tests pass on CI
• Documentation reflects the changes where applicable
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

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CodSpeed Performance Report

Merging #11161 will not alter performance

_{Comparing discriminated-union-errors (bca5c56) with main (7485725)}

Summary

✅ 46 untouched benchmarks

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Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
pydantic/_internal
_discriminated_union.py					385
Project Total

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[sydney-runkle]

Nice, this seems like a good improvement.

If we raised this in more than 2 places I'd suggest we save the message somewhere and use it through a variable, but I think this is fine for the 2 usage approach.

[Viicos]

If we raised this in more than 2 places I'd suggest we save the message somewhere and use it through a variable, but I think this is fine for the 2 usage approach.

As I mentioned, a refactor would get rid of the two places, so probably best to go this way in the future.