feat(google-vertexai): support Gemini propertyOrdering in withStructu…

[thomasrowe5]

…redOutput (#33720)

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[thomasrowe5]

✨ Summary
Adds support for propertyOrdering in Gemini structured output schemas within withStructuredOutput().
This enables developers to define or preserve ordered field generation when using Vertex AI Gemini models.
🧩 Context
Google’s Gemini structured output schema supports a propertyOrdering array that defines the order in which model output fields are generated.
Without this, Gemini ignores ordering, leading to undesirable outputs for order-sensitive tasks (e.g. reasoning traces or step-based scribing).
🛠️ Implementation
Added helper withPropertyOrdering() in chat_models.ts to automatically infer property ordering from the provided schema when missing.
Integrated the helper into schemaToGeminiParameters() path within ChatGoogleBase.withStructuredOutput.
No other behavioral changes.
✅ Testing
All google-common provider tests pass.
Full monorepo test suite passes except a single flaky math_utils timeout in @langchain/core (unrelated).
📚 Example
const schema = z.object({
reason: z.string(),
decision: z.string(),
});
const model = new ChatGoogleVertexAI().withStructuredOutput(schema);
→ Automatically constructs propertyOrdering: ["reason", "decision"].
🔗 Related
Fixes #33720
PR authored by @thomasrowe5