Add rustdoc comments to all public items

Document all public structs, enums, traits, functions, fields, and
variants across the library. Adds crate-level documentation with a
usage example, module-level docs, and API reference links where
applicable. Also fixes four bare URL warnings in existing doc comments.

src/error.rs

@@ -1,21 +1,39 @@
+//! Error types for the Google Gemini client.
+
 use std::fmt::Display;
 use tokio_util::codec::LinesCodecError;
 
 use crate::types;
 
+/// A type alias for `Result<T, error::Error>`.
 pub type Result<T> = std::result::Result<T, Error>;
 
+/// Errors that can occur when using the Gemini client.
 #[derive(Debug)]
 pub enum Error {
+    /// An environment variable required for configuration was missing or invalid.
     Env(std::env::VarError),
+    /// An HTTP transport error from the underlying `reqwest` client.
     HttpClient(reqwest::Error),
+    /// A JSON serialization or deserialization error.
     Serde(serde_json::Error),
+    /// A structured error returned by the Vertex AI API.
     VertexError(types::VertexApiError),
+    /// A structured error returned by the Gemini API.
     GeminiError(types::GeminiApiError),
+    /// The API response contained no candidate completions.
     NoCandidatesError,
+    /// An error occurred while decoding the SSE event stream.
     EventSourceError(LinesCodecError),
+    /// The SSE event stream closed unexpectedly.
     EventSourceClosedError,
-    GenericApiError { status: u16, body: String },
+    /// An API error that could not be parsed into a structured error type.
+    GenericApiError {
+        /// The HTTP status code.
+        status: u16,
+        /// The raw response body.
+        body: String,
+    },
 }
 
 impl Display for Error {