Allow injecting fetch into TypeScript clients

[meoyawn]

Summary

• add an optional fetch implementation to generated TypeScript client options
• dispatch final interceptor requests through the injected implementation
• reuse injected fetch for SSE reconnects
• update generated golden files

Root cause

Generated clients called globalThis.fetch directly, preventing callers from supplying a custom transport and bypassing it during SSE reconnects.

Impact

Consumers can now pass fetch through ClientOptions; existing consumers continue using globalThis.fetch by default.

Closes #1.

Validation

• task check in packages/oasmith
• root monorepo task check