Add JsonPatch support for Chat, Responses, and Embeddings

[ShivangiReja]

This PR adds JsonPatch support to Chat, Responses, and Embeddings so callers can:

• Pass additional properties on requests
• Read unmodeled properties from responses via the Patch bag.

Motivation

• OpenAI exposes fast-moving, model-specific knobs that aren’t on the typed surface.
• Developers need a way to round-trip those fields without waiting on spec/SDK updates.

Examples:

Responses

// You can use the Patch property to set additional properties in the request
ResponseCreationOptions options = new();
options.Patch.Set("$.reasoning.effort"u8, "high");
options.Patch.Set("$.text.verbosity"u8, "medium");

OpenAIResponse response = client.CreateResponse("What is the answer to the ultimate question of life, the universe, and everything?", options);

// You can also read additional properties back from the response
var effort = response.Patch.GetString("$.reasoning.effort"u8);
var verbosity = response.Patch.GetString("$.text.verbosity"u8);
Console.WriteLine($"effort={effort}, verbosity={verbosity}");

Fixes: #259

[m-nash]

The new visitor tests are a great start thanks for adding them

[sheng-jie]

Great job