The output fixing parser isn't working with Gemini 2.5 flash.

[carlows]

Describe the bug
The output fixing parser isn't working with Gemini 2.5 flash.

As you described in the readme, google models require a different format:

messages = [
  { role: "system", content: "You are a helpful assistant." },
  { role: "user", content: "What's the weather like today?" }
  # Google Gemini and Google VertexAI expect messages in a different format:
  # { role: "user", parts: [{ text: "why is the sky blue?" }]}
]

However, the OutputFixingParser does not take that into account: https://github.com/patterns-ai-core/langchainrb/blob/main/lib/langchain/output_parsers/output_fixing_parser.rb#L50-L55

To Reproduce
Using the output fixing parser:

parser = Langchain::OutputParsers::StructuredOutputParser.from_json_schema('some_schema.json'))
Langchain::OutputParsers::OutputFixingParser.from_llm(llm:, parser:)

Raises an error when using the model gemini 2.5-flash.

Expected behavior
The output fixing parser should work with google/gemini models

Terminal commands & output
Commands you used and the terminal output.

Screenshots
If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• OS: [e.g. OS X, Linux, Ubuntu, Windows]
• Ruby version [3.3]
• Langchain.rb version [0.19.5]

Additional context

[carlows]

The error from google:

{"error"=>{"code"=>400, "message"=>"Invalid JSON payload received. Unknown name "content" at 'contents[0]': Cannot find field.", "status"=>"INVALID_ARGUMENT", "details"=>[{"@type"=>"type.googleapis.com/google.rpc.BadRequest", "fieldViolations"=>[{"field"=>"contents[0]", "description"=>"Invalid JSON payload received. Unknown name "content" at 'contents[0]': Cannot find field."}]}]}}

[carlows]

Opened a PR with a suggestion, but maybe there's a way to do it with the UnifiedParameters? Or I'm missing something obvious...