Function parse_connection_string 

pub fn parse_connection_string(s: &str) -> Result<(String, String), CosmosError>

Parse an Azure Cosmos DB connection string.

Expected format: AccountEndpoint=https://acct.documents.azure.com:443/;AccountKey=<base64-key>;

Returns (endpoint, master_key). Trailing semicolon is optional. Additional fields (e.g., DatabaseName=) are ignored.