1fix: prevent duplicate TCP bootstrap server creation

[lymerin]

Fixes #6735.

What is changed

TcpProxySelectorDataHandler previously used a non-atomic check-then-act sequence:

inCache -> createBootstrapServer -> cache

Concurrent selector synchronization events could both observe a cache miss and start bootstrap servers for the same selector. This could cause port binding failures or leave an overwritten server and its event-loop resources without lifecycle management.

This change:

• adds per-selector single-flight creation in TcpBootstrapFactory;
• ensures concurrent creation requests for the same selector wait for one in-flight creation;
• performs server startup outside ConcurrentHashMap atomic callbacks and without a global lifecycle lock;
• allows different selectors to be created and removed independently;
• uses putIfAbsent when publishing the started server and shuts down a losing instance if another server was already cached;
• routes TcpProxySelectorDataHandler through the atomic creation and removal methods;
• preserves the existing public factory methods for compatibility;
• rolls back LoopResources when bootstrap startup fails;
• makes TcpBootstrapServer.shutdown() idempotent and preserves shutdown failures using suppressed exceptions.

The change is limited to the duplicate bootstrap creation and related resource lifecycle described in #6735. Existing selector configuration update behavior is unchanged.

Tests

Added or extended tests covering:

• concurrent creation of the same selector creates only one server;
• a failed creation can be retried;
• waiting callers receive the original creation failure;
• slow shutdown of one selector does not block removal of another selector;
• bootstrap startup failure disposes its loop resources;
• repeated shutdown is idempotent;
• failures from both server and loop-resource disposal are preserved.

The following targeted Maven build passed locally:

mvn -pl shenyu-protocol/shenyu-protocol-tcp,shenyu-plugin/shenyu-plugin-proxy/shenyu-plugin-tcp \
    -am \
    -Dtest=TcpBootstrapFactoryTest,TcpBootstrapServerTest,TcpProxySelectorDataHandlerTest \
    -Dsurefire.failIfNoSpecifiedTests=false \
    test

Results:

• TcpBootstrapServerTest: 11 tests passed
• TcpBootstrapFactoryTest: 4 tests passed
• TcpProxySelectorDataHandlerTest: 3 tests passed
• Checkstyle: no violations

Checklist

• I have read the [contribution guidelines](https://shenyu.apache.org/community/contributor-guide).
• I have submitted test cases that cover the changes.
• I have run the complete ./mvnw clean install -Dmaven.javadoc.skip=true build.