feat: add reasoning_effort support to ChatGroq

[YJack0000]

Summary

The Python langchain-groq package supports reasoning_effort as a first-class constructor parameter and passes it to the Groq API via _default_params. The JS/TS @langchain/groq package does not — there is no way to set reasoning_effort through the constructor or call options.

Current behavior (JS @langchain/groq)

ChatGroq has reasoningFormat (maps to reasoning_format) but no reasoningEffort field. The invocationParams() method constructs the request body explicitly and does not forward arbitrary kwargs, so there's no workaround via extra fields.

// This does NOT work — reasoningEffort is silently ignored
const model = new ChatGroq({
  model: 'openai/gpt-oss-120b',
  reasoningEffort: 'low', // ← not a recognized field
});

Expected behavior

ChatGroq should accept reasoningEffort (or reasoning_effort) as a constructor param and include it in the API request body, matching the Python implementation:

# Python langchain-groq — this works
model = ChatGroq(
    model="openai/gpt-oss-120b",
    reasoning_effort="low",
)

Proposed changes

1. Add reasoningEffort field to ChatGroqInput interface
2. Add class field with type from groq-sdk
3. Store it on the class instance (in constructor)
4. Include it in invocationParams() output
5. Add to CREATE_PARAMS_BASE_CALL_KEYS for call options support
6. Add to lc_serialized_keys for serialization

Reference

• Python langchain-groq already has this: reasoning_effort field passed via _default_params
• Groq API docs: https://console.groq.com/docs/reasoning#options-for-reasoning-effort
• Supported models: openai/gpt-oss-20b, openai/gpt-oss-120b, qwen/qwen3-32b