fix(model): preserve SSE data alongside comments and ignored fields #397

_handle_generate_stream_response drops an entire SSE event when its first line is a comment:

: heartbeat
data: {"choices":[{"delta":{"content":"Answer"},"finish_reason":"stop"}]}

This should produce Answer with finish reason stop.
Unknown fields before data and standalone id: events are also rejected.
Still present in the #396 merge (a91e6b4a).
#396 closed this issue on merge; a separate fix is still required.

• Ignore comments line by line, unknown fields and events without data, following the SSE rules.
• Preserve data around comments, usage, [DONE] and response cleanup across fragmented input.
• Cover existing raw JSON, custom delimiters, encodings and EOF behavior when changing the parser.

Update the existing regression tests on main to expect the corrected output.
From #394 and the review in #396.