Make CompositeInputStream safe when close() races with an in-progress read

[LI123456mo]

What is the purpose of the change?

Fixes #16458

CompositeInputStream is not thread-safe. When a client connection drops during HTTP/2 streaming, close() can run on the I/O thread while a worker thread is still reading from the same stream. That can surface a raw IOException ("Stream already closed") from a read, and available() can keep reporting data after the stream has been closed.

Brief changelog

• Made addInputStream, both read methods, available and close synchronized, so close() waits for an in-flight read instead of closing streams underneath it.
• Added a closed flag: available() returns 0 once closed, and an IOException from an already-closed underlying stream is treated as end of stream instead of being surfaced.

Verifying this change

Added CompositeInputStreamConcurrencyTest (5 tests):

• read(byte[]) and read() interrupted by close() report end of stream (no raw exception)
• already-buffered data is still delivered when close() happens during a read
• available() returns 0 while close() is in progress
• close() waits for a read in progress (two-thread test)

Before the change, 4 of these 5 fail; the fifth is a regression guard that passes either way. After the change, all pass.

Checklist

• Make sure there is a GitHub_issue field for the change.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Write necessary unit-test to verify your logic correction.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 46.15385% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.91%. Comparing base (6054104) to head (15f1566).
⚠️ Report is 12 commits behind head on 3.3.

Files with missing lines	Patch %	Lines
...he/dubbo/remoting/http12/CompositeInputStream.java	46.15%	10 Missing and 4 partials ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##                3.3   #16471       +/-   ##
=============================================
+ Coverage     36.93%   60.91%   +23.98%     
+ Complexity    11752       15    -11737     
=============================================
  Files          1952     1953        +1     
  Lines         89249    89291       +42     
  Branches      13389    13477       +88     
=============================================
+ Hits          32961    54389    +21428     
+ Misses        51710    29307    -22403     
- Partials       4578     5595     +1017     

Flag	Coverage Δ
integration-tests-java21	32.13% <23.07%> (+0.05%)	⬆️
integration-tests-java8	32.21% <23.07%> (+<0.01%)	⬆️
samples-tests-java21	32.17% <23.07%> (-0.01%)	⬇️
samples-tests-java8	29.83% <23.07%> (+0.03%)	⬆️
unit-tests-java11	59.16% <46.15%> (?)
unit-tests-java17	58.66% <46.15%> (?)
unit-tests-java21	58.65% <46.15%> (?)
unit-tests-java25	58.62% <46.15%> (?)
unit-tests-java8	59.19% <46.15%> (?)

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