feat: expose Display trait on Amount and Psbt types

[shivv23]

Description

Add #[uniffi::export(Display)] and implement the Display trait on the Amount and Psbt types, exposing them to the FFI bindings (Kotlin, Swift, Python).

Notes to the reviewers

This follows the same pattern used by other types in the codebase (FeeRate, Script, Address, Transaction, WalletEvent, etc.) that already expose Display to bindings. Part of issue #1020.

Documentation

• bitcoin — Amount and Psbt already implement Display upstream.

Changelog

changelog: added

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing
• I've added exactly one changelog:* label
• I've linked the relevant upstream docs or specs above

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

[j-kon]

Nice small addition, and this seems consistent with the Display exports already used for types like FeeRate, Script, Address, and Transaction.

One small question: should this PR add a focused test for the new binding-facing Display behavior? Since this exposes Display through UniFFI, it would be useful to assert that Amount and Psbt stringify as expected, especially because the PR checklist marks tests as added but the diff only changes bitcoin.rs.

[shivv23]

Hey @j-kon , I should have included tests with the initial commit. I've pushed test_amount_display and test_psbt_display to cover the new Display behavior for both types.

Regarding the changelog label — I have changelog: added in the PR body but it seems the label itself needs to be applied by someone with triage access. Could you add it?

Thanks for the review!