fix: emit complete SSE chunks immediately

[OllieinCanada]

Fixes #301

Summary

Emit complete SSE events immediately when their blank-line delimiter ends at a network chunk boundary.

Root cause

The delimiter scanner skipped the final two-byte candidate, so \n\n and \r\r at the end of a chunk were not recognized until more data arrived. The line decoder also retained a terminal carriage return even after the SSE chunker had identified a complete event block.

Fix

• scan the final possible two-byte delimiter position
• flush the line decoder after each complete SSE event block
• preserve existing support for LF, CR, and CRLF delimiters

Tests

A deterministic in-memory async stream verifies that events terminated by \n\n, \r\r, and \r\n\r\n are emitted before the reader asks for another network chunk.

Validation:

• focused regression tests: 3 passed
• offline unit suite: 552 passed
• TypeScript typecheck passed
• Biome check passed for the regression test
• git diff --check passed

No external API or credentials were used.

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Note

Low Risk
Internal streaming parser fix with focused regression tests; no API or auth changes.

Overview
Fixes delayed SSE delivery when a complete event’s blank-line delimiter (\n\n, \r\r, or \r\n\r\n) lands at the end of a network chunk.

findDoubleNewlineIndex now scans through the last possible two-byte delimiter position (buffer.length - 1 instead of buffer.length - 2), so trailing \n\n / \r\r at a chunk edge are recognized immediately.

_iterSSEMessages flushes the LineDecoder after each complete SSE block from iterSSEChunks, so a terminal carriage return is not held until the next read.

Adds regression tests in streaming-utils.test.ts that assert an event is yielded before the stream reader is asked for a second chunk for all three delimiter styles.

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