Skip blocks while marking computed nils unknown

[jar-b]

Relates #483
Relates hashicorp/terraform-provider-corner#89
Relates hashicorp/terraform-provider-aws#27857 (acceptance tests pass after this fix)

MarkComputedNilsAsUnknown now passes block values through unmodified, rather than throwing an error.

I extended the single large test object to add block attributes, but I can also split into individual test cases if that'd be helpful. Original issue is reproduced with the updated test case and the fix commented out:

$ go test -count=1 -run "^TestMarkComputedNilsAsUnknown$" ./internal/fwserver/...
--- FAIL: TestMarkComputedNilsAsUnknown (0.00s)
    server_planresourcechange_test.go:283: Unexpected error: AttributeName("block-nil-optional-computed"): couldn't find attribute in resource schema: path leads to block, not an attribute
FAIL
FAIL    github.com/hashicorp/terraform-plugin-framework/internal/fwserver       0.353s
FAIL

---

Edit: #552 included the change to ignore block type errors, so this PR has been updated to just extend test coverage.

[hashicorp-cla]

All committers have signed the CLA.

[bflad]

Hi @jar-b 👋 Thank you for submitting this change. This appears to overlap with #552 which was recently merged, however this change does include additional TestMarkComputedNilsAsUnknown testing. If you're willing or able, I'd suggest rebasing this against main and dropping the (now duplicate) changelog entry.

[bflad]

Looks good to me, thanks so much @jar-b 🚀

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