fix: Update the README following the module re-organization

[fjuma]

Description

This PR updates the README following the module re-org changes.

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• Ensure the tests pass
• Appropriate READMEs were updated (if necessary)

[gemini-code-assist]

Summary of Changes

Hello @fjuma, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request primarily focuses on updating the project's README.md to align with recent module re-organization within the A2A Java SDK. The changes aim to provide clearer and more accurate guidance for users on integrating both server and client components of the SDK, alongside a minor dependency cleanup in the TCK module.

Highlights

• Documentation Update: The README.md has been significantly updated to reflect recent module re-organization within the A2A Java SDK, ensuring the documentation accurately guides users on setting up server and client dependencies.
• Server Dependency Guidance Streamlined: Instructions for adding A2A Java SDK server dependencies have been streamlined and clarified. The documentation now offers a clear choice between a2a-java-sdk-server-jakarta and a2a-java-sdk-server-quarkus and consolidates previously separate steps into a single, comprehensive section.
• Client Dependency Clarity: The section detailing the A2A Java SDK client dependency has been re-positioned and simplified, making it clearer how to include the a2a-java-sdk-client in a project.
• Dependency Clean-up in TCK: The tck/pom.xml file has been updated to remove a direct Maven dependency on a2a-java-sdk-spec. This suggests that this dependency is either no longer required directly or is now transitively included by other dependencies.

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[gemini-code-assist]

Code Review

This pull request updates the README.md to align with recent module reorganizations and cleans up a redundant dependency in the tck/pom.xml. The changes significantly improve the clarity and structure of the documentation. I've suggested a couple of minor additions to the README to guide users on versioning, as the current change removes previous guidance without replacing it.

[gemini-code-assist]

The dependency snippet uses a Maven property ${io.a2a.sdk.version} without explaining where this version comes from. This could be confusing for users, especially since the previous version of the README provided specific guidance on versioning.

To improve clarity and usability, I suggest adding a note that explains users need to replace this placeholder with a specific version and provides links to find the latest releases.

Suggested change

	Add **one** of the following dependencies to your project:
	Add **one** of the following dependencies to your project:
	<!-- Place this comment at the location where the note should be added -->
	```xml
	<dependency>
	<groupId>io.a2a.sdk</groupId>
	<artifactId>a2a-java-sdk-server-jakarta</artifactId>
	<version>${io.a2a.sdk.version}</version>
	</dependency>

Note: Replace ${io.a2a.sdk.version} with the desired SDK version. You can find the latest available versions on Maven Central or in the project's releases.

[gemini-code-assist]

Similar to the server dependency, the client dependency snippet uses the ${io.a2a.sdk.version} property without context. It would be beneficial for users to have guidance on how to find and specify the correct version.

Please consider adding a note here as well.

Suggested change

	To make use of the Java `A2AClient`, simply add the following dependency:
	To make use of the Java `A2AClient`, simply add the following dependency:
	<!-- Place this comment at the location where the note should be added -->
	```xml
	<dependency>
	<groupId>io.a2a.sdk</groupId>
	<artifactId>a2a-java-sdk-client</artifactId>
	<version>${io.a2a.sdk.version}</version>
	</dependency>

Note: Replace ${io.a2a.sdk.version} with the desired SDK version. You can find the latest available versions on Maven Central or in the project's releases.