[Tech Debt] Update stale OpenAI default model gpt-3.5-turbo to gpt-4o-mini #905

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Description

The OpenAI Chat Completions chat model defaults to gpt-3.5-turbo when the caller omits model. This is a legacy OpenAI model — it has been superseded by cheaper, more capable small models, and OpenAI itself steers new integrations away from it. A user who takes the default today silently gets an outdated model.

The stale default lives in both language runtimes and the docs, so a user hits it regardless of which API they use:

• Java — OpenAICompletionsSetup.DEFAULT_MODEL (integrations/chat-models/openai/src/main/java/org/apache/flink/agents/integrations/chatmodels/openai/OpenAICompletionsSetup.java:61)
• Python — DEFAULT_OPENAI_MODEL (python/flink_agents/integrations/chat_models/openai/openai_chat_model.py:38)
• Docs — the OpenAICompletionsSetup parameter tables for both languages (docs/content/docs/development/chat_models.md)

The Java Javadoc example already uses gpt-4o-mini (OpenAICompletionsSetup.java:45), so the code base is already inconsistent with its own default.

Proposed fix: update the default in both runtimes and the docs to gpt-4o-mini — the current low-cost successor to gpt-3.5-turbo, and the value the Javadoc example already demonstrates. This keeps a sensible zero-config default while dropping the legacy model, and it keeps Java and Python in parity.

Are you willing to submit a PR?

• I'm willing to submit a PR!