refactor(heightindex): remove safety of byte modifications for Put & Get

[DracoLi]

Why this should be merged

This simplifies HeightIndex implementations by removing the need to defensive copy. The current use case for database.HeightIndex should never modify the data after Put or Get, so the extra copying is unnecessary.

resolves #4453

How this works

Removed slice.Clone calls in Put and Get operations.
The database.HeightIndex interface was updated to indicate that byte slices are not safe to modify.

How this was tested

Unit tests

Need to be documented in RELEASES.md?

No

[Copilot]

Pull Request Overview

This PR refactors the HeightIndex database interface and its implementations to remove defensive copying of byte slices, simplifying the code under the assumption that callers will not modify data after Put or Get operations.

Key Changes:

• Removed slices.Clone calls from Put and Get operations across implementations
• Updated interface documentation to clarify that byte slices are not safe to modify
• Removed tests that verified defensive copying behavior

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
database/database.go	Updated HeightIndex interface documentation to indicate byte slices are not safe to modify
database/heightindexdb/memdb/database.go	Removed slices.Clone calls from in-memory database implementation
database/heightindexdb/dbtest/dbtest.go	Removed tests verifying that returned data is independent of modifications
x/blockdb/cache_db.go	Removed slices.Clone calls from cache database implementation
x/blockdb/cache_db_test.go	Removed tests that verified defensive copying behavior

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