fix: handle missing parts field in gemini flash responses (#2272)

Gemini Flash 2.0 & 2.5 sometimes omits the parts field when returning
very short responses (e.g., single words), causing deserialization to
fail with `missing field parts` error.
 
This pr adds `#[serde(default)]` to `Content.parts` to gracefully handle
these cases by defaulting to an empty vector.
 
 Fixes #1749 
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----

> [!IMPORTANT]
> Adds `#[serde(default)]` to `Content.parts` in `types.rs` to handle
missing `parts` field, fixing deserialization errors for short
responses.
> 
>   - **Behavior**:
> - Adds `#[serde(default)]` to `Content.parts` in `types.rs` to handle
missing `parts` field by defaulting to an empty vector.
> - Fixes deserialization errors for short responses from Gemini Flash
2.0 & 2.5.
>   - **Misc**:
>     - Fixes issue #1749.
> 
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Co-authored-by: Doral Pipeline <pipelines@doralhw.org>

Author: jmsegrev

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

@@ -234,6 +234,7 @@ pub struct Candidate {
 #[serde(rename_all = "camelCase")]
 pub struct Content {
     pub role: Option<String>,
+    #[serde(default)]
     pub parts: Vec<Part>,
 }