Standardize JdkHttpSender shutdown to await executor termination

[thswlsqls]

Related to #8280 — continues #8495

Description

• shutdown() used graceful executorService.shutdown(), which does not interrupt workers sleeping in the retry backoff, and never awaited termination — the result code completed while export threads were still running.
• It also called HttpClient.close(), which blocks until all operations complete, on the caller's thread.
• Now calls shutdownNow(), awaits termination (5s, warning on timeout), and closes the client on a daemon thread before completing the result.
• This mirrors the sibling HttpSender implementations OkHttpGrpcSender (the reference named in the issue) and OkHttpHttpSender. OkHttp's cancelAll() / evictAll() have no java.net.http equivalent; shutdownNow() plus close() cover that role.
• Close failures still propagate through the result code; unmanaged executors still return immediately.

Testing done

• Added JdkHttpSenderTest#shutdown_managedExecutor_awaitsTermination (executor terminated once the result succeeds) and #shutdown_nonManagedExecutor_doesNotShutDownExecutor (completes immediately, injected executor left running).
• Existing testShutdown / testShutdownException cover close-exception propagation on the new async path.
• ./gradlew :exporters:sender:jdk:check — 13 tests passed.
• No apidiff or CHANGELOG.md entry: the japicmp-excluded internal package only.

[codecov]

Codecov Report

❌ Patch coverage is 77.77778% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.59%. Comparing base (b86f040) to head (c861861).

Files with missing lines	Patch %	Lines
...ry/exporter/sender/jdk/internal/JdkHttpSender.java	77.77%	3 Missing and 1 partial ⚠️

❌ Your patch check has failed because the patch coverage (77.77%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8627      +/-   ##
============================================
- Coverage     91.60%   91.59%   -0.01%     
- Complexity    10343    10346       +3     
============================================
  Files          1013     1013              
  Lines         27352    27368      +16     
  Branches       3215     3216       +1     
============================================
+ Hits          25057    25069      +12     
- Misses         1567     1570       +3     
- Partials        728      729       +1     

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