cleanup(sync): remove obsolete zcashd trailing hash handling

[jvff]

Motivation

Closes #11091

Zebra discards the final hash from FindBlocks responses on Mainnet and Testnet as a workaround for a deprecated zcashd behavior that could append an unrelated hash.

The workaround was originally added in #991 for a zcashd bug that combined responses. Since zcashd has been deprecated, Zebra no longer needs to discard otherwise valid trailing hashes.

Solution

Remove trailing-hash stripping from both sync stages:

• obtain_tips() now considers the complete FindBlocks response on every network.
• extend_tips() retains the final new hash after removing the expected overlap hash.

The general overlap handling in extend_tips() remains unchanged.

Update the existing sync test vectors to omit their artificial sacrificial trailing hashes. The existing download assertions now verify that the final valid hash is retained and downloaded.

Tests

• cargo fmt --all -- --check
• cargo nextest run -p zebrad 'components::sync::tests::vectors'
  • 10 tests passed
• git diff --check

Specifications & References

• Always drop the final hash in peer responses #991 originally introduced the trailing-hash workaround for a zcashd bug.

Follow-up Work

Handle valid singleton FindBlocks responses independently. Removing the trailing-hash workaround alone does not allow the syncer to construct a CheckedTip from a single unknown hash.

AI Disclosure

• No AI tools were used in this PR
• AI tools were used: OpenAI Codex was used to investigate the sync behavior and history, implement the change, update tests, run validation, and draft the PR description.

PR Checklist

• The PR title follows conventional commits format: type(scope): description
• The PR follows the contribution guidelines.
• This change was discussed in an issue or with the team beforehand.
• The solution is tested.
• The documentation and changelogs are up to date.

[Copilot]

Pull request overview

Removes obsolete zcashd trailing-hash stripping from Zebra’s block synchronization flow.

Changes:

• Retains complete FindBlocks responses in obtain_tips() and extend_tips().
• Updates sync vectors to verify final valid hashes are downloaded.
• No consensus or serialization behavior changes.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
zebrad/src/components/sync.rs	Removes trailing-hash filtering from both sync stages.
zebrad/src/components/sync/tests/vectors.rs	Removes sacrificial hashes and validates retained trailing hashes.