docs(esplora): explain prevout trust assumptions

[h55n]

Description

Refs #2343

Documents that Esplora sync and full_scan add server-reported previous outputs as floating txouts without checking them against the full parent transaction. Explains how fee and sent/received numbers can depend on those values, or on scripts a server reports, when the parent transaction is absent, including inputs that belong to other parties. Adds a trust-assumptions section to the Esplora README and links the sync and full_scan docs on both the blocking and async extension traits to it.

This changes documentation only. It does not add parent-transaction verification.

Notes to the reviewers

Doc-only, 33 added lines in 3 files. I used "Refs" because I am not sure whether you also want the behaviour changed (verifying parents) rather than documented. Happy to adjust the wording or scope to whatever you prefer on #2343.
Behaviour described in the docs is checked by a separate local harness: a forged floating txout value changes the computed fee, and fetching the full actual parent transaction restores the correct fee.

Changelog notice

Documented the trust assumptions of server-reported previous outputs in bdk_esplora.

Checklists

All Submissions:

• I followed the contribution guidelines

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing (doc-only change, checked with doctest and docs build)
• I'm linking the issue being fixed by this PR (Refs bdk_esplora: document that prevout values inserted during sync are unverified server-reported data #2343)

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