feat: add default noop providers for AgentCard and -Executor

[kabir]

This avoids CDI injection problems when building an application with the Quarkus plugin, and still makes people have to implement them to run their application

Description

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Fixes #213

[gemini-code-assist]

Summary of Changes

Hello @kabir, 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 introduces default, non-functional CDI bean producers for AgentCard and AgentExecutor within the Quarkus application context. The primary goal is to prevent build-time injection failures in Quarkus applications while still requiring developers to supply their own functional implementations for these core components at runtime.

Highlights

• CDI Injection Fix: I've added a new class, DefaultProducers.java, to provide default 'no-op' CDI beans for AgentCard and AgentExecutor. This addresses a known issue (Issue #213) where Quarkus applications could encounter injection failures if these beans were not explicitly provided.
• Default Bean Implementation: The new default beans are annotated with Quarkus's @DefaultBean, ensuring they are used only if no other concrete implementation is available. If these default beans are ever invoked, they will throw an IllegalStateException, clearly guiding developers to provide their own custom implementations.
• Guidance for Developers: The exception messages thrown by the default producers explicitly inform the user that they need to provide their own AgentCard and AgentExecutor implementations as CDI beans, and that their custom beans will automatically take precedence.

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Code Review

This pull request introduces default CDI producers for AgentCard and AgentExecutor to prevent injection issues in Quarkus applications when these beans are not yet implemented by the user. The approach of providing default beans that throw an IllegalStateException at runtime is a good pattern to guide developers. The implementation is straightforward.