test(registry): add coverage parity for get_registered_page

[Mhazii]

Summary

This PR brings get_registered_page test coverage in line with the existing paginated registry APIs and clarifies when each pagination endpoint should be used.

The goal is to ensure all three pagination APIs behave consistently across common registry scenarios and to reduce the risk of indexers relying on an insufficiently tested endpoint.

Changes

• Added parity tests for get_registered_page covering:
  • Empty registry
  • Single registered entry
  • Middle entry removal
  • Last-page boundaries
  • Pagination behavior across multiple pages
• Added equivalent coverage for relevant get_registered_paginated and get_public_paginated scenarios where needed.
• Verified admin vs public access behavior.
• Verified the verified field is returned consistently.
• Verified behavior while the contract is paused.
• Added an API comparison table to docs/ABI.md explaining which pagination endpoint should be used and when.
• Reviewed the existing implementations for behavioral differences.
• Included regression tests for any bugs discovered during the parity review.

Testing

• Ran registered page tests.
• Ran pagination-related tests.
• Verified empty and populated registry responses.
• Verified pagination across first, middle, and last pages.
• Verified removed entries do not cause users to be skipped unexpectedly.
• Verified admin/public access differences.
• Verified verified field behavior.
• Verified paused-contract behavior.
• Confirmed existing API results remain unchanged except where a tested bugfix was required.

Acceptance Criteria

• get_registered_page has coverage parity with the paginated variants.
• Empty, single-entry, middle-removal, and last-page scenarios are covered.
• ABI documentation includes a clear which-API-to-use table.
• Admin/public behavior is covered.
• verified and pause behavior are covered.
• No existing API behavior was changed unnecessarily.
• Any discovered bugs are covered by regression tests.
• Pagination tests pass successfully.

Validation

cargo test registered_page
cargo test paginat

closes #299 
closes #300 
closes #301 
closes #302 

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