Test/attester registry edge cases and error docs

[aji70]

Summary

Adds test coverage for two previously-unconsidered edge cases in the attester-registry contract (self-referential admin/contract addresses as
attesters, and calling propose_admin twice before accept_admin), and improves the doc comments on the AllowlistFull and
InvalidRegistryWiring errors across both the attester-registry and attestation-registry contracts to give integrators actionable
remediation guidance.

Changes

#161 — Add edge-case tests for zero/self-referential addresses in attester-registry

• Admin-address and contract-address add_attester tests, behavior documented per test with a one-line comment

#162 — Add a test for calling propose_admin twice before accept_admin

• Overwrite-vs-reject behavior determined empirically and tested, doc comment added above propose_admin

#163 — Improve the AllowlistFull error doc comment with actionable remediation

• Remediation paths (set_max_attesters / remove_attester) added to the doc comment, docs/error-codes.md synced

#164 — Improve the InvalidRegistryWiring error doc comment with remediation guidance

• Remediation guidance added to the doc comment in attestation-registry, docs/error-codes.md synced

Closes #161, Closes #162, Closes #163, Closes #164

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