fix(esplora): deduplicate missing txids in fetch_txs_with_outpoints

[phrwlk]

Description

Previously fetch_txs_with_outpoints collected spend txids into a Vec, so the same txid could be pushed multiple times when one transaction spent several input outpoints. This caused redundant get_tx_info calls to Esplora for the same transaction, wasting network and CPU without changing the resulting TxUpdate.

Use HashSet<Txid> for missing_txs in both async and blocking fetch_txs_with_outpoints, so each txid is only requested once while keeping the observable behaviour of SyncResponse / TxUpdate unchanged.

Changelog notice

### Changed
- Use `HashSet` instead of `Vec` to track `missing_txs` in bdk_esplora, it deduplicates txids.

Checklists

All Submissions:

• I followed the contribution guidelines

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

[evanlinjin]

cACK

Looks good, thanks for fixing this.

Just a minor thing before merging - could you combine the two commits and include the scope?

I.e.

fix(esplora): deduplicate missing txids in fetch_txs_with_outpoints

[phrwlk]

cACK

Looks good, thanks for fixing this.

Just a minor thing before merging - could you combine the two commits and include the scope?

I.e.

fix(esplora): deduplicate missing txids in fetch_txs_with_outpoints

Did it

[evanlinjin]

ACK 787e07c

[oleonardolima]

cACK 787e07c

[oleonardolima]

Can you update the PR description to follow the template, and also add the changelog changes in it ?

[notmandatory]

This will also need a rebase also since I merged #2123.

[oleonardolima]

ACK 3b6b3ba

I did rebase and update the description.

[luisschwab]

ACK 3b6b3ba