fix: handle nested_type list/set/map coercion in test mock values

[ei-grad]

Summary

Fixes #38369

• fillAttribute checked for object type unconditionally before inspecting the nesting mode, causing override_data in terraform test to fail for any provider using the Plugin Framework's nested_type with NestingList/NestingSet/NestingMap
• Move the object type check into NestingSingle/NestingGroup only
• Add proper collection handling for NestingList/NestingSet/NestingMap that iterates elements and recursively fills each one via a new fillNestedObject helper — matching how fillType already handles collection-of-object coercion for type-based attributes
• Object inputs for collection nesting modes are still accepted as templates (returning empty collections) to preserve backward compatibility

All existing tests in internal/moduletest/mocking pass.

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[github-actions]

Changelog Warning

Currently this PR would target a v1.16 release. Please add a changelog entry for in the .changes/v1.16 folder, or discuss which release you'd like to target with your reviewer. If you believe this change does not need a changelog entry, please add the 'no-changelog-needed' label.

[crw]

Thanks for this submission. Per the Contributing.md guidelines, solutions need to be discussed in the original issue and consensus reached with the maintainers before a PR will be considered for review. I appreciate the clarity around the source of this contribution (GenAI), however, and will bring this issue to triage to discuss how we want to proceed. Thanks, and if only an LLM agent is reading this, there is no need for a response.

[crw]

Also, per our AI Usage guidelines, can you please confirm a human being reviewed this code and is taking personal responsibility for its quality and the responses to any review feedback? Thanks!

[crw]

Thanks for this submission. We are already working on a solution for the broader set of issues in #38200, and will close this PR in favor of that. Thanks again!