[release/6.0] Throw on unsupported types in src gen

[github-actions]

Backport of #58775 to release/6.0

/cc @eiriktsarpalis@steveharter

Customer Impact

Types unsupported by sourcegen are currently being silently serialized (typically as {}). This can result in invalid data accidentally being returned in API responses or written to databases.

Testing

Tests have been added affirming the new throwing behavior.

Risk

Low. While this is adding more code to the sourcegen implementation, these additions are relatively small and straightforward.

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Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, to please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

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Issue Details

---

Backport of #58775 to release/6.0

/cc @eiriktsarpalis@steveharter

Customer Impact

Testing

Risk

Author:	github-actions[bot]
Assignees:	-
Labels:	area-System.Text.Json, new-api-needs-documentation
Milestone:	-

[Anipik]

cc @danmoseley

[danmoseley]

is this a functional change for the non-generator scenario?

[danmoseley]

Approved. Fixes fairly mainstream scenario in this new feature. Code change essentially limited to generators.

[danmoseley]

If we consider future fixes to the generator, it would reduce risk if it was possible to make zero changes to the non generator codepaths.