test(examples): Add credential-free documentation example harness

[thedriftofwords]

Summary

• Adds a credential-free harness that validates registered Terraform documentation examples with terraform fmt -check, terraform init -backend=false, and terraform validate.
• Exposes the command as make -f GNUmakefile test-examples and runs it in CI.
• Documents the command and registration workflow in examples/README.md.

Dependency

Draft until coreweave/terraform-provider-coreweave#304 merges.

This branch is rebased on PR #304 so the harness validates the updated bucket policy examples, including required_providers and named principal ARNs. After #304 lands, rebase this branch onto main and mark ready for review.

Related: DOCS-3002

Test plan

• Run make -f GNUmakefile test-examples
• Run go test ./coreweave/object_storage
• Confirm no .terraform/ or lockfiles are left under examples/
• CI test-examples job passes after rebase onto main post-docs(object-storage): Scope bucket policy examples by org condition #304

Design notes

• Default harness is credential-free and stops at validate (no plan or apply).
• Examples are copied to temporary directories before validation so source trees stay clean.
• Register additional examples by adding paths to tools/test-examples.sh.

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