feat(a2a): route remote reasoning events through middleware

[wmh123wmh1]

Summary

• add middleware() and middlewares() configuration to A2aAgent.Builder
• keep registered middlewares in the same order convention as ReActAgent
• bridge the A2A reasoning lifecycle (start, chunk, end) into MiddlewareBase.onReasoning()
• preserve the existing Hook API and deprecated stream() behavior
• add focused builder and event-order compatibility tests

Intentionally limited scope

This is a small draft implementation to validate the integration direction with maintainers.

• No streamEvents() API is added.
• onActing(), onModelCall(), and onSystemPrompt() are unchanged.
• Existing Hook dispatch and legacy response behavior are retained.
• The A2A bridge currently exposes the remote lifecycle entries as CustomEvents to the reasoning middleware; the returned middleware event stream is consumed for lifecycle operators, while response rewriting remains out of scope for this first pass.

Refs #2612

Tests

• mvn -pl agentscope-extensions/agentscope-extensions-protocol/agentscope-extensions-a2a/agentscope-extensions-a2a-client -am -DskipITs test
  • agentscope-core: 2316 tests, 0 failures, 9 skipped
  • A2A client: 112 tests, 0 failures

[codecov]

Codecov Report

❌ Patch coverage is 79.48718% with 8 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...scope/core/a2a/agent/event/ClientEventContext.java	72.41%	1 Missing and 7 partials ⚠️

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