Function is_retryable_camel_error 

pub fn is_retryable_camel_error(err: &CamelError) -> bool

Classify a CamelError as retryable (transient network/IO errors).

Retryable variants:

• CamelError::Io — I/O errors (connection refused, DNS, etc.)
• CamelError::ProcessorError whose message contains the literal [TRANSIENT] marker (used by gRPC and other components to flag retryable-by-classification errors).
• CamelError::ProcessorErrorWithSource whose message contains the [TRANSIENT] marker (same semantics, preserves source error chain).

Non-retryable variants:

• CamelError::Config, CamelError::TypeConversionFailed, CamelError::Stopped, CamelError::EndpointCreationFailed, CamelError::ChannelClosed — permanent failures