Fix PrometheusHttpServerBuilder copy constructor dropping the default handler

[thswlsqls]

Fixes #8618

Description

• The package-private PrometheusHttpServerBuilder copy constructor copied 8 of the builder's 9 fields, omitting defaultHandler and silently losing an HttpHandler set via setDefaultHandler().
• build() stores a defensive copy on the server and toBuilder() copies that again, so toBuilder() users lose the handler despite the documented "same configuration as this instance" contract. The first build() is unaffected — it passes defaultHandler from the field directly.
• Added the missing assignment in field-declaration order, matching the adjacent authenticator line: an identical @Nullable, setter-only field passed to HTTPServer.builder(). The field arrived in Make /metrics the only Prometheus metrics endpoint #6476 without a copy-constructor line; memoryMode had the same omission, fixed later in Enable reusuable_data memory mode by default #6799.

Testing done

• Extended PrometheusHttpServerTest#toBuilder: the new assertion fails with expected: <handler> but was: null before the fix, passes after.
• ./gradlew :exporters:prometheus:check — 191 tests plus 1 testJpms test pass.
• CHANGELOG.md ## Unreleased entry added. No apidiff: signatures unchanged, alpha module untracked in docs/apidiffs/.
• Not run: *IT and GraalVM native-image tests (outside check).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.60%. Comparing base (63ebd94) to head (8d2370a).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8619      +/-   ##
============================================
- Coverage     91.63%   91.60%   -0.03%     
- Complexity    10326    10343      +17     
============================================
  Files          1013     1013              
  Lines         27295    27353      +58     
  Branches       3205     3215      +10     
============================================
+ Hits          25011    25058      +47     
- Misses         1558     1567       +9     
- Partials        726      728       +2     

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