Fix/chain position ffi serialization

[kumulynja]

Description

While using bdk-dart I encountered the following error after having broadcasted a transaction and wanting to list the transactions which resulted in app crashes:

unexpected enum: UniFfi::UnexpectedEnumCase    ParallelWaitError(2 errors): UniFfi::UnexpectedEnumCase    #0      FfiConverterChainPosition.read (package:bdk_dart/bdk.dart:5622:9)

ChainPosition was writing garbage bytes (0x00da7f10 in this specific case) instead of valid enum discriminants (1 or 2).

Tracked it down with Claude Code to the following cause in bdk-ffi:

The From implementation called .into() on a borrowed ChainPosition<&BdkConfirmationBlockTime>. This caused a lifetime mismatch with the From trait expecting an owned value, resulting in undefined behavior during FFI serialization.

Solution implemented in this PR:

Explicitly pattern match on the borrowed chain_position and construct the ChainPosition enum correctly, fixing the "invalid enum index" error when calling Wallet.transactions().

Tested successfully with bdk-dart and the crashing wallet.

Notes to the reviewers

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

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

[kumulynja]

I added test cases to bdk-ffi first, but they passed without the fix as well, since the bug appears in the uniffi serialization, not on Rust-level conversions. So I removed them again and it would be better to add tests for it in the languages of the bindings themselves.

[kumulynja]

I made some wrong conclusions, BdkConfirmationBlockTime is not borrowed, and the problem I encountered seems to be solved by the following: bitcoindevkit/bdk-dart#47. So closing the issue.