[release/6.0] Support JsonConverterFactory with src-gen

[github-actions]

Backport of #58398 to release/6.0

/cc @steveharter

Customer Impact

Support for JsonConverterFactory is currently broken in source generators. It has the potential to block customers from migrating existing reflection-based serialization DTOs to source gen.

Testing

Added testing that validate the broken scenaria.

Risk

Low to moderate. Accommodating factories requires a moderate amount of refactoring in product code.

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Issue Details

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Backport of #58398 to release/6.0

/cc @steveharter

Customer Impact

Testing

Risk

Author:	github-actions[bot]
Assignees:	-
Labels:	area-System.Text.Json
Milestone:	-

[Anipik]

cc @danmoseley

[layomia]

LGTM, source/generated code changes are sound; test coverage is robust and addresses this core scenario (JsonConverterFactory + src-gen) for 6.0. cc @danmoseley

[danmoseley]

Given the churn and that it's arguably a documentable limitation, I doubt this would make the RC2 bar (starting 9/14). But I'm OK taking this now as it will increase adoption of this new feature. As discussed offline, I appreciate the comprehensive tests but since there's significant churn here I would prefer to get another pair of eyes reviewing it before merging.

[danmoseley]

Oh, I see @layomia did that. OK, we can merge when green.