fix: support multi-megabyte SSE events

[kokojacket]

Summary

• raise the shared SSE scanner limit from 2 MiB to 16 MiB while keeping the 128 KiB initial buffer
• reuse the configured scanner for streaming, aggregation, completion, usage, text, and rate-limit parsing
• add a regression test covering a 3 MiB response.completed event

Why

Image-generation responses can contain multi-megabyte SSE data lines. The previous 2 MiB scanner limit aborted otherwise successful upstream streams with bufio.Scanner: token too long.

Tests

• go test ./... -count=1