feat: introduce BlockHeaderStorage.get_tip helper

[xdustinface]

Introduces get_tip() -> Option<HashesBlockHeader> in BlockHeaderStorage. This consolidates two-step pattern of get_tip_height() followed by get_header(tip_height) into a single get_tip() call. I didn't adjust any of the other places (except for one) where it's currently possible to be used since they are in the "old" sync code and this PR here is more like a preparation to be used by the sync rewrite.

Based on:

• refactor: add test_utils modules #347

Summary by CodeRabbit

• Refactor
  • Optimized block tip hash retrieval by eliminating intermediate lookups, reducing computational overhead and improving query responsiveness
  • Enhanced block storage interface with streamlined access to current block header tip information
  • Simplified internal control flow while maintaining complete backward compatibility with existing APIs

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[coderabbitai]

📝 Walkthrough

Walkthrough

The changes refactor the tip hash retrieval mechanism by introducing a new get_tip() method on the storage API that returns a BlockHeaderTip struct containing height, header, and hash. This simplifies client-side code by eliminating sequential height and header lookups, replacing them with a single storage call.

Changes

Cohort / File(s)	Summary
Storage API Enhancement dash-spv/src/storage/blocks.rs, dash-spv/src/storage/mod.rs	Added new public BlockHeaderTip struct with height, header, and hash fields. Extended BlockHeaderStorage trait with async get_tip() method. Implemented get_tip() in PersistentBlockHeaderStorage to retrieve current tip and construct BlockHeaderTip. Exposed BlockHeaderTip in storage module imports. Includes test validation.
Client Refactoring dash-spv/src/client/core.rs	Updated tip_hash() implementation to directly call storage.get_tip() instead of fetching tip height then header separately. Preserves return type and API contract while streamlining internal logic.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Poem

A simpler path, the storage gleams,
No more chasing height-header dreams!
BlockHeaderTip hops into sight,
One call replaces three—what delight! 🐰✨
The client code now runs so light!

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 15.38% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: introducing a new BlockHeaderStorage.get_tip helper method that consolidates tip retrieval logic.

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✨ Finishing touches

• 📝 Generate docstrings

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