fix(envoy-client): bound the shared WebSocket writer queue

Problem

The native and WASM envoy-client transports feed the single actor-to-engine WebSocket writer through an unbounded mpsc channel. If producers outpace the socket, queued protocol messages can grow without bound.

This is a shared transport issue, not specific to SSE. The queue carries HTTP request/response chunks alongside WebSocket traffic, events, SQLite, and other envoy messages, so an SSE-only limit would leave the generic failure mode in place.

Desired behavior

• Bound queued writer memory across all message types.
• Apply backpressure or fail fast without blocking an actor's serialized event loop.
• Preserve control-message progress and disconnect/reconnect semantics.
• Define fair accounting for differently sized messages; a message-count-only bound is insufficient.
• Add saturation tests for native and WASM transports, including HTTP streaming, WebSockets, events, and SQLite.
• Add queue-depth/queued-byte and overload metrics.

Relevant code

• engine/sdks/rust/envoy-client/src/connection/native.rs
• engine/sdks/rust/envoy-client/src/connection/wasm.rs
• engine/sdks/rust/envoy-client/src/connection/mod.rs

Discovered while reviewing the HTTP/SSE streaming change.