fix(client): accept larger SSE events

[kitlangton]

Summary

• raise the generated Promise client's SSE event limit from 1 MiB to 16 MiB
• report oversized events as SseEventTooLarge instead of MalformedResponse
• cover fragmented valid events below the limit and oversized events above it

Context

The v2 event schema currently permits multi-megabyte payloads. In practice, session.tool.success can include large structured.output values and session.input.admitted can include inline base64 attachments.

The 1 MiB Promise-client limit caused every connected TUI to reject the same valid global event as malformed and reconnect simultaneously. This change is the immediate compatibility fix; #36441 tracks scoped subscriptions, bounded event projections, stream lifecycle, and fan-out observability for 2.0.

Validation

• bun run --cwd packages/client test
• bun run --cwd packages/httpapi-codegen test
• bun run --cwd packages/client typecheck
• bun run --cwd packages/httpapi-codegen typecheck
• bun run --cwd packages/client check:generated
• full repository typecheck via the pre-push hook