fix(fetch): ignore SSE comment lines instead of ending the stream

[pacocartones]

Description

parseSseLine returned { done: true } for the : ping keep-alive comment, and streamSse uses that flag to break out of the loop that drains the line buffer.

When a server or proxy flushes the comment in the same chunk as the data: events that follow (common with TCP/TLS coalescing and gateways such as LiteLLM or nginx), those events stay in the buffer. If the stream ends at that point, the trailing-buffer fallback sees a value starting with \n, does not match the data: prefix, and yields nothing — so the remainder of the model response is silently dropped, with no error and no log.

Per the SSE spec, any line starting with a colon is a comment and must be ignored. This changes the branch to match any :-prefixed line and return { done: false }, so the buffer keeps draining.

streamSse is the shared helper behind most providers (OpenAI, Anthropic, Deepseek, Moonshot, VertexAI, LlamaStack, Inception, SiliconFlow, HuggingFace, Cohere), so this affects any of them talking to a server that sends SSE comments.

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Checklist

• I've read the contributing guide
• The relevant docs, if any, have been updated or created
• The relevant tests, if any, have been updated or created

Screen recording or screenshot

Not applicable — no user-facing UI change. The behaviour is covered by the unit test below.

Tests

Added ignores SSE comment lines that share a chunk with data events to packages/fetch/src/stream.test.ts. It pushes a single chunk containing : ping followed by two data: events and data: [DONE].

Before the fix the test fails with expected [] to deeply equal [ { foo: 'bar' }, { baz: 42 } ] — the entire response is lost. After the fix both events are yielded.

npx vitest run in packages/fetch: 6 files, 83 tests passing.