fix: change output selection and dust threshold logic

[famouswizard]

Description

• change now always uses Keychain::Internal instead of .last() - avoids random external change or panics when no internal descriptor is present.
• DrainWeights now matches the change address type (taproot, segwit, legacy) for accurate fee and dust estimates.
• switched dust comparison from > to >= so exact dust-sized change isn’t lost to fees.

Notes to the reviewers

small but important cleanup for more predictable change handling and fee estimation. Worth double-checking test coverage for non-taproot outputs.

Changelog notice

fix incorrect change keychain selection, fee estimation, and dust threshold comparison.

Checklists

All Submissions:

• I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)

New Features:

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

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR