feat(ollama): add support for native structured outputs

[jonghwanhyeon]

This PR aligns the ChatOllama implementation with LangChain Python by enhancing the withStructuredOutput() method to support native structured outputs.

Key Changes

• withStructuredOutput() now accepts a method option with the following values:
  • "jsonSchema": Passes the JSON Schema directly to the format parameter for native structured outputs.
  • "jsonMode": Sets the format to "json" to enforce valid JSON generation.
  • "functionCalling" (default): Uses tool calling to extract structured outputs.

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