Preserve original error chains (reqwest, serde_json, etc.) in SdkError
via Arc<dyn Error>-backed #[source] fields on Network, RequestTimeout,
Stream, and Configuration variants. This makes production debugging of
network/TLS/DNS issues easier since error reporters can now walk the
full chain. Serde-compatible via #[serde(skip)] — message string still
carries the text for serialized forms.
Also add a `type` field to ToolCall (defaulting to "function") so
non-function tool types from providers won't be silently mishandled.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>