test(relay): exercise operator listener delivery over HTTP

[jsibbison-square]

Summary

• Remove the single-implementation DeliveryStore and DeliveryTransport traits and their mocks.
• Send operator-listener notifications directly through reqwest and the existing database methods.
• Replace the hand-rolled HTTP parser with local axum delivery tests. The handler verifies NIP-98 against the received URL, POST method, and body. Tests assert persisted success, retry, terminal failure, redirect, and unroutable-claim behavior.

Follow-up to review point 4 on #7793.

Validation

• cargo test -p buzz-relay --lib operator_listener::tests:: -- --include-ignored with the repository's configured local database: 7 passed.
• . ./bin/activate-hermit && just ci: exit 1. Formatting and clippy passed; buzz-acp tests failed on three environment-sensitive cases (session_new_forwards_complete_git_block_without_duplicate_names, allowed_respond_to_full_path_unset_allows_all, test_session_policy_default_is_channel).
• . ./bin/activate-hermit && just test: exit 1. The same three buzz-acp cases failed, plus buzz-db's p0_pool_acquisitions_use_typed_operation_pairs_without_other source check and two workspace integration tests (oauth_missing_token_uses_configured_model_then_retries_discovery, non_auth_discovery_failure_uses_configured_model_without_caching_fallback). These failures also appear in the prior follow-up's logs.

Draft pending agent review and human behavior verification.

[github-actions]

🔐 Codex Security Review

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