reject is_instance inputs based on their type, not the mode

[davidhewitt]

Change Summary

This changes the way the is-instance validator is disabled to reject inputs based on whether they originated from Python or JSON, not based on the parse mode.

This allows is_instance validation to be possible when coming from JSON, provided you use a before or wrapping validator function (etc) which coerces the input to Python first.

Related issue number

Ref pydantic/pydantic#6573

Checklist

• Unit tests for the changes exist
• Documentation reflects the changes where applicable
• Pydantic tests pass with this pydantic-core (except for expected changes)
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

Selected Reviewer: @samuelcolvin

[codspeed-hq]

CodSpeed Performance Report

Merging #765 will not alter performance

_{Comparing dh/pyd-issue-6571 (0be0d24) with main (a3b9788)}

Summary

✅ 126 untouched benchmarks

[adriangb]

Can you make a pydantic PR to prove this works?

[davidhewitt]

Try rebase on #767 and we should know :)

[davidhewitt]

test-pydantic-integration here is all green, please review