[Pipe] Drop builtin sinks synchronously

[Caideyipi]

Description

Make builtin sink cleanup synchronous when a pipe is dropped.

Behavior

• Builtin sinks, including the subscription sink, acquire the sink operation lock, discard buffered events, and close the connector on the caller thread.
• External sinks keep the existing timeout-bounded asynchronous cleanup behavior so third-party connector implementations cannot block pipe drop indefinitely.
• Sink classification is centralized in BuiltinPipePlugin.BUILTIN_SINKS, including connector names, sink names, and legacy aliases.

Tests

Added regression coverage for:

• builtin sink discard waiting for an in-progress connection retry;
• external sink discard remaining non-blocking during connection retry;
• builtin sink close waiting for connector close;
• external sink close retaining its timeout behavior.

---

This PR has:

• been self-reviewed.
  • concurrent read
  • concurrent write
  • concurrent read and write
• added comments explaining the why and the intent of the code wherever it would not be obvious.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage.
• added integration tests.
• been tested in a test IoTDB cluster.

---

Key changed/added classes (or packages if there are too many classes) in this PR

• PipeSinkSubtask
• PipeSinkSubtaskManager
• SubscriptionSinkSubtask
• BuiltinPipePlugin
• PipeSinkSubtaskTest

[codecov]

Codecov Report

❌ Patch coverage is 95.65217% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 42.82%. Comparing base (a05c63e) to head (420ecd7).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
.../pipe/agent/task/subtask/sink/PipeSinkSubtask.java	86.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master   #18646   +/-   ##
=========================================
  Coverage     42.82%   42.82%           
  Complexity      442      442           
=========================================
  Files          5451     5451           
  Lines        395425   395467   +42     
  Branches      51805    51807    +2     
=========================================
+ Hits         169349   169378   +29     
- Misses       226076   226089   +13     

☔ 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.