docs(esplora): replace misleading tx-cache TODO with explanation

[Arowolokehinde]

Description

fetch_txs_with_outpoints carried a misleading // TODO: We should maintain a tx cache (like we do with Electrum). comment in both async_ext.rs and blocking_ext.rs. Electrum needs a cache because its history call returns txids only, forcing a follow-up fetch per tx. Esplora's scan endpoint already returns full tx bodies, so there's no equivalent fetch for a cache to skip — and inserted_txs already dedupes against what the spk phase fetched earlier in the same sync.

This PR replaces the TODO with a comment explaining why, and why the obvious "fix" — using get_tx_status instead of get_tx_info for cache hits — is unsafe: unknown txids return HTTP 200 {"confirmed":false}, indistinguishable from a real unconfirmed tx, so evicted transactions would never be evicted.

Fixes #2260

Notes to the reviewers

The TODO already cost a contributor a 600-line PR (#2254, closing #2250) implementing exactly this cache, which was caught in review for the get_tx_status eviction bug described above. This comment exists to stop that from being rediscovered a third time.

Changelog notice

Removed a misleading TODO in fetch_txs_with_outpoints suggesting a tx cache be added; replaced with a comment explaining why the Electrum-style cache pattern does not apply to Esplora.

Checklists

All Submissions:

• I followed the contribution guidelines

• I'm linking the issue being fixed by this PR

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.36%. Comparing base (456f9b7) to head (1aa2ece).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2261   +/-   ##
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  Coverage   78.36%   78.36%           
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  Files          30       30           
  Lines        5945     5945           
  Branches      281      281           
=======================================
  Hits         4659     4659           
  Misses       1210     1210           
  Partials       76       76           

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