Validate add_foreign_utxo inputs at method boundary

[reez]

Description

This PR aligns bdk-ffi TxBuilder::add_foreign_utxo with upstream bdk_wallet boundary validation behavior.

• Validate foreign input immediately in add_foreign_utxo (instead of deferring to finish), behavior now matches method’s documented error semantics
• Update test_add_foreign_utxo_missing_witness_data to assert failure at add_foreign_utxo and remove wallet sync/recipient setup that is no longer needed for this boundary test

Notes to the reviewers

Directly related to comment: #928 (comment)

I implemented a broader version of my original suggestion by aligning add_foreign_utxo with full upstream boundary validation, so all documented input errors are returned immediately instead of being deferred to finish().

Documentation

• bdk_wallet
• bitcoin
• uniffi
• Other:

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 a changelog in the next release tracking issue (see example)
• 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

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
  • feat: expose only_witness_utxo and add_foreign_utxo on TxBuilder #928 (comment)