fix(esplora): stop paging when the server does not advance the cursor

[luhrhenz]

Description

Fixes #2339.

bdk_esplora pages each script pubkey's history by passing the previous page's last txid as the cursor, and stops only on a page of fewer than 25 txs. A server that ignores the cursor returns the same full page forever, so sync/full_scan never returns and memory grows without bound.

The loop now records every cursor it pages from and returns InvalidResponse when a full page ends on one it has already seen. This catches both a repeated page and a cycle (A -> B -> A). Applied to both async_ext.rs and blocking_ext.rs.

Notes to the reviewers

• It returns an error rather than breaking, so that sync never reports a truncated history as success.
• There is no page cap, so long histories aren't truncated. A correct server never repeats a cursor, so the check can't trigger on one.
• The regression tests use a small TcpListener stub in tests/common that always returns the same page, since TestEnv can't fake a misbehaving server. tokio's time dev-dep feature is for the async test's timeout.

Changelog notice

• Fixed bdk_esplora's pagination loop hanging when a chain source returns a page that does not advance the last_seen cursor.

Checklists

All Submissions:

• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.67%. Comparing base (7f7f653) to head (19ba4e3).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2361      +/-   ##
==========================================
+ Coverage   79.53%   79.67%   +0.14%     
==========================================
  Files          31       31              
  Lines        6141     6149       +8     
  Branches      290      292       +2     
==========================================
+ Hits         4884     4899      +15     
+ Misses       1182     1178       -4     
+ Partials       75       72       -3     

Flag	Coverage Δ
rust	79.67% <100.00%> (+0.14%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[evanlinjin]

This only catches a cursor that repeats straight away. A server that cycles (A → B → A, e.g. one that maps the cursor to a wrong page) still loops forever. Tracking every cursor seen catches any finite cycle for about the same amount of code:

let mut seen = HashSet::new();
// ...
let cursor = txs.last().map(|tx| tx.txid);
// ...
if !seen.insert(cursor) { return Err(...) }
last_seen = cursor;

Let's do this instead as it's more thorough.

[evanlinjin]

ACK 19ba4e3

I had to update the PR description as it was too wordy.