fix: prevent false success for rejected on-chain sends

[ovitrif]

Parent: #712
Source: Pubky marketplace Bitkit team brief
Counterpart: synonymdev/bitkit-android#1211

Scope

• Reproduce the stale Electrum-tip/non-final transaction case from the brief.
• Determine whether ownership lies in Bitkit, LDK Node, or the Electrum integration, and link any upstream issue.
• Make the send result distinguish accepted broadcast from local submission when the backend rejects or drops the transaction.

Acceptance criteria

• The reproduced non-final transaction is not shown as Bitcoin Sent.
• A clean-wallet accepted transaction still completes normally.
• Regression coverage records the expected signal.

[ovitrif]

Root cause is upstream in synonymdev/ldk-node#112.

Both iOS and Android pin LDK Node 0.7.0-rc.63. Its on-chain send API returns a Txid after enqueueing the transaction, before Electrum/Esplora/bitcoind processes the broadcast. Backend rejection cannot reach either mobile caller. The Electrum path additionally logs a successfully joined task as a successful broadcast without inspecting the nested transaction_broadcast result, which explains the reported “successful Electrum broadcast” for the rejected non-final transaction.

The upstream issue includes the stale-tip reproduction, exact pinned/current revisions, mobile call-site impact, and required accepted-versus-submitted result contract.