Enforce strict JSON Schema compliance to prevent OpenAI 400 Bad Request errors

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Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: Tool calls fail with 400 Bad Request due to strict JSON Schema violations (OpenAI) #1265 (Tool calls fail with 400 Bad Request due to strict JSON Schema violations (OpenAI) #1265)
• Related: Tool calls fail with 400 Bad Request due to strict JSON Schema violations (OpenAI) #1265

2. Or, if no issue exists, describe the change:

Problem:
When using strict OpenAI-compatible providers like Groq, empty function arguments or responses are serialized as null or omitted by ChatCompletionsRequest. Furthermore, zero-argument tools fail to declare a parameters schema object. This breaks strict JSON schema parsers, causing them to drop conversation history, which leads to 400 Bad Request exceptions due to models hallucinating raw XML <function> tags. JSON Schema enum types are also improperly serialized in uppercase.

Solution:

• Added a custom schemaNormalizerModule to the ObjectMapper in ChatCompletionsRequest.java to force Type enums to serialize in lowercase (e.g., "string").
• Added enforceJsonObject to ChatCompletionsCommon.java to guarantee that empty arguments and function response payloads fallback to "{}" instead of null or raw strings.
• Updated ChatCompletionsRequest.java to explicitly inject a {"type":"object", "properties":{}} parameters schema for zero-argument functions instead of omitting it.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Passed: mvn test
Added testFromLlmRequest_withEmptyFunctionArguments to ChatCompletionsRequestTest.java to explicitly test "{}" serialization for zero-argument tools. Updated testFromLlmRequest_withFunctionResponse expectations.

Manual End-to-End (E2E) Tests:

Wired the Google native chat completion client into the external model-prism project (PR #1199). Ran the integration through all the demo tests covering all main ADK features. The applications successfully execute multi-turn multi-tool conversations using strict JSON schema endpoints without throwing 400 Bad Request due to context loss.

Checklist

• I have read the CONTRIBUTING.md document.
• My pull request contains a single commit.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context

This change unlocks robust support for the rapidly growing ecosystem of OpenAI-compatible endpoints that enforce strict JSON Schema validation.