Add explicit WebSocket receive caps

[ryanfowler]

Summary

• Set explicit 16 MiB frame and message limits on WebSocket receives instead of relying on tokio-tungstenite defaults.
• Map oversized inbound messages to a clear fetch diagnostic.
• Extend WebSocket test helpers for 64-bit frame lengths and add coverage for messages just below and above the cap.
• Document the inbound 16 MiB limit in the WebSocket docs and CLI reference.

Testing

• cargo fmt
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --all-features
• cargo test --all-features --test cli --test formatting --test grpc --test http --test network --test terminal --test update --test websocket -- --test-threads=1