Fix chunking strategy serialization and field visibility

[nikessel]

Fix chunking strategy serialization and field visibility

Description

This PR fixes three issues with chunking strategies in the vector stores API:

1. Incorrect serialization format for static chunking strategy
2. Incorrect casing in enum variant serialization ("Static" vs "static")
3. Private fields preventing users from setting chunking parameters

Changes

Serialization Fix

The OpenAI API expects the static chunking configuration to be nested under a "static" field, but the current implementation serializes it directly. This causes the error:

"Missing required parameter: 'chunking_strategy.static'."

Additionally, the enum variants were serializing with incorrect casing, causing:

"Invalid value: 'Static'. Supported values are: 'auto' and 'static'."

Changed from:

#[derive(Clone, Serialize, Debug, Deserialize, PartialEq, Default)]
#[serde(tag = "type")]
pub enum VectorStoreChunkingStrategy {
    #[default]
    Auto,
    Static(StaticChunkingStrategy),
}

To:

#[derive(Clone, Serialize, Debug, Deserialize, PartialEq, Default)]
#[serde(tag = "type")]
pub enum VectorStoreChunkingStrategy {
    #[default]
    #[serde(rename = "auto")]
    Auto,
    #[serde(rename = "static")]
    Static {
        #[serde(rename = "static")]
        config: StaticChunkingStrategy,
    },
}

Field Visibility

Made fields in StaticChunkingStrategy public to allow users to configure chunking parameters:

#[derive(Clone, Serialize, Debug, Deserialize, PartialEq, Default)]
pub struct StaticChunkingStrategy {
    pub max_chunk_size_tokens: u16,
    pub chunk_overlap_tokens: u16,
}

[64bit]

Thank you for detailed description and PR!