Fix http2 client dynamic table RC #10820

[danielkec]

Fixes #10820

[Copilot]

Pull Request Overview

This PR adds thread-safety to HTTP/2 header processing in the client by synchronizing access to the shared inbound dynamic table across concurrent streams. The main changes include centralizing header reading logic and adding a test to verify correct behavior under concurrent stream multiplexing with many unique headers.

• Introduced a thread-safe readHeaders method in Http2ClientConnection using ReentrantLock to protect the shared inbound dynamic table
• Refactored Http2ClientStream.readHeaders to delegate to the connection-level method
• Added a new test case multiplexParallelStreamsGetWithUniqueHeaders that exercises concurrent header processing with 1000 unique headers per stream

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
webclient/http2/src/main/java/io/helidon/webclient/http2/Http2ClientConnection.java	Adds ReentrantLock for inbound dynamic table synchronization and introduces a new readHeaders method to centralize thread-safe header processing
webclient/http2/src/main/java/io/helidon/webclient/http2/Http2ClientStream.java	Refactors header reading to use the new connection-level readHeaders method for thread-safe access to the dynamic table
webclient/http2/src/test/java/io/helidon/webclient/http2/Http2WebClientTest.java	Adds new test endpoint and test method to verify concurrent header processing, includes copyright year update and code formatting improvements

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