Parser for openai must have created as an optional field (#1778)

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> [!IMPORTANT]
> Makes `created` field in `ChatCompletionGeneric` optional during
deserialization to handle missing values in OpenAI responses.
> 
>   - **Deserialization**:
> - Makes `created` field in `ChatCompletionGeneric` optional by adding
`#[serde(default, ...)]`, allowing missing `created` in OpenAI responses
to deserialize as `None`.
>   - **Scope**:
> - Change is limited to `types.rs`, only affects
`ChatCompletionGeneric` struct.
> 
> <sup>This description was created by </sup>[<img alt="Ellipsis"
src="https://img.shields.io/badge/Ellipsis-blue?color=175173">](https://www.ellipsis.dev?ref=BoundaryML%2Fbaml&utm_source=github&utm_medium=referral)<sup>
for 81ef224. It will automatically
update as commits are pushed.</sup>

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Author: hellovai

engine/baml-runtime/src/internal/llm_client/primitive/openai/types.rs

@@ -14,7 +14,7 @@ pub struct ChatCompletionGeneric<C> {
     /// A list of chat completion choices. Can be more than one if `n` is greater than 1.s
     pub choices: Vec<C>,
     /// The Unix timestamp (in seconds) of when the chat completion was created.
-    #[serde(deserialize_with = "deserialize_float_to_u32")]
+    #[serde(default, deserialize_with = "deserialize_float_to_u32")]
     pub created: Option<u32>,
     /// The model used for the chat completion.
     pub model: String,