fix: add getter method for McpPingResponse#result (#2971)

The result field in McpPingResponse is private and lacks a corresponding getter method, which will cause the result field to be lost during Jackson serialization.

Issue

Closes #2970

Change

Add a getter method for McpPingResponse#result

General checklist

There are no breaking changes
I have added unit and/or integration tests for my change
The tests cover both positive and negative cases
I have manually run all the unit and integration tests in the module I have added/changed, and they are all green
I have manually run all the unit and integration tests in the core and main modules, and they are all green
I have added/updated the documentation
I have added an example in the examples repo (only for “big” features)
I have added/updated Spring Boot starter(s) (if applicable)

Checklist for adding new maven module

I have added my new module in the root pom.xml and langchain4j-bom/pom.xml

Checklist for adding new embedding store integration

I have added a {NameOfIntegration}EmbeddingStoreIT that extends from either EmbeddingStoreIT or EmbeddingStoreWithFilteringIT
I have added a {NameOfIntegration}EmbeddingStoreRemovalIT that extends from EmbeddingStoreWithRemovalIT

Checklist for changing existing embedding store integration

I have manually verified that the {NameOfIntegration}EmbeddingStore works correctly with the data persisted using the latest released version of LangChain4j

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