fix(client): handle stream start rejection in subscribe

[shivanshgupta365]

Summary

• handle the fire-and-forget start promise created by ShapeStream.subscribe
• keep fatal errors delivered through the subscriber error callback without also producing a process-level unhandled rejection
• add a regression test for the Node/Bun failure path

Fixes #4736

Validation

• 35 ShapeStream unit tests passed
• TypeScript client typecheck passed
• TypeScript client build passed
• ESLint, Prettier, and lint-staged passed