feat: add gpt-5.3-chat-latest model support (#9746)

Add OpenAI's GPT-5.3 Chat model (128K context, 16K output) with pricing
specs, and remove the "chat" keyword filter so chat variants are listed.

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Co-authored-by: Letta Code <noreply@letta.com>

tests/model_settings/openai-gpt-5.3-chat-latest.json

@@ -0,0 +1,11 @@
+{
+  "handle": "openai/gpt-5.3-chat-latest",
+  "model_settings": {
+    "provider_type": "openai",
+    "max_output_tokens": 4096,
+    "parallel_tool_calls": false,
+    "reasoning": {
+      "reasoning_effort": "minimal"
+    }
+  }
+}