fix(inbound): retry duplicate block advertisers

[evan-forbes]

Motivation

Closes: N/A

When multiple peers advertised the same gossiped block, Zebra previously kept only the first advertiser. A malicious first advertiser could race honest peers, then timeout or return malformed data, causing the gossip download to fail even though other peers had also advertised the block.

Solution

• Record bounded fallback advertisers for active gossiped block downloads.
• Retry eligible source failures with the next fallback source.
• Avoid fallback retries for consensus-invalid matching blocks, including production-shaped RouterError::Block verifier errors.
• Add regression tests for fallback retry after malformed data and no retry after consensus-invalid data.

Validation

• cargo test -p zebrad components::inbound
• cargo fmt --all -- --check

In this environment the zebrad tests were run with CXXFLAGS='-include cstdint' to work around a local librocksdb-sys C++ include issue.

AI Disclosure

Used OpenAI Codex to audit, implement, and test this change.

[ValarDragon]

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[chatgpt-codex-connector]

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