fix(state): Limit the retrieval of note commitment subtrees

[upbqdn]

Motivation

Close #7708.

The issue in #7708 was that when retrieving the subtrees from the finalized and non-finalized state, we always added all subtrees from the non-finalized state to the response without paying attention to the requested limit.

Solution

• Apply the given limit when retrieving subtrees.
• In non-release builds, make sure the limit applies.
• Add tests that test if we combine the subtrees from finalized and non-finalized states correctly. In order to implement the tests:
  • Derive Default for Chain.
  • Derive Default for Sapling & Orchard note commitment tree Node.
  • Allow direct insertions of subtrees into Chain for tests.
  • Set the visibility of DiskWriteBatch to pub(super).

Unrelated Clean-ups

• Use default() for subtree roots in other tests.
• Clarify some docs.

Reviewer Checklist

• Will the PR name make sense to users?
  • Does it need extra CHANGELOG info? (new features, breaking changes, large changes)
• Are the PR labels correct?
• Does the code do what the ticket and PR says?
  • Does it change concurrent code, unsafe code, or consensus rules?
• How do you know it works? Does it have tests?

[arya2]

This looks great, thank you!