fix!: address NITs in PR #361

[iajoiner]

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• [x] The PR title and commit messages adhere to guidelines here: https://github.com/spaceandtimelabs/sxt-proof-of-sql/blob/main/CONTRIBUTING.md. In particular ! is used if and only if at least one breaking change has been introduced.
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Rationale for this change

Since I had auto-merge on NITs didn't get addressed. I promised to file a PR to resolve them today and hence I do now.

What changes are included in this PR?

• remove UnimplementedTestAccessor which is no longer used
• remove ProofPlan::is_empty which is only used in VerifiableQueryResult by inlining
• make code more compact in query_proof_test.rs and verifiable_query_result_test.rs

  Existing tests should pass.

[iajoiner]

Note that this is a breaking change since ProofPlan::is_empty was public

[iajoiner]

https://github.com/spaceandtimelabs/sxt-proof-of-sql/actions/runs/11786476589/job/32829840774?pr=367 This doesn't work. I'm undoing the change to compactify the code base.

[github-actions[bot]]

:tada: This PR is included in version 0.44.0 :tada:

The release is available on GitHub release

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