feat(rust): exclude non-streaming types from prost generation (#7097)

Exclude non-streaming protobuf message and enum types from generated
prost output files when hybrid streaming generation is enabled.

The filterModelToStreaming helper now collects all non-well-known-type
IDs that are unreachable from bidirectional streaming RPCs. These unused
type IDs are passed to the sidekick rust_prost generator and configured
with prost_build extern_path mappings to
crate::prost::unused::NonexistentType, preventing prost from emitting
unused struct definitions and unused output files.

For #6835