test: cover TxUpdate::map_anchors

[mubashirrao1122]

This PR adds test coverage for TxUpdate::map_anchors.

It verifies that anchor values are correctly transformed while txids and other
fields (such as seen_ats) are preserved.

Tests:

• cargo test -p bdk_core --test test_tx_update

[Copilot]

Pull request overview

Adds unit tests in bdk_core to exercise TxUpdate behaviors, primarily TxUpdate::map_anchors, ensuring anchors are transformed while other update fields remain intact.

Changes:

• Add TxUpdate::is_empty coverage for populated txs and seen_ats.
• Add coverage that TxUpdate::extend makes an update non-empty when extending with non-empty data.
• Add a map_anchors test verifying anchor transformation and preservation of txid and seen_ats.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

The inner use bitcoin::hashes::Hash; inside test_map_anchors_transforms_anchor_type_and_preserves_txid is redundant because the trait is already imported at module scope. This will trigger unused_imports and fail CI since clippy runs with -D warnings. Remove the inner use (or drop the module-level import and import the trait only where needed, but avoid duplicating it).

Suggested change

use bitcoin::hashes::Hash;

[mubashirrao1122]

Removed the redundant import to avoid unused_imports under clippy.