Replace per-chunk parse_sse_frames with SseParser holding raw bytes
across chunks: reassembles frames split at TCP boundaries (the review's
silently-losing case), decodes UTF-8 per complete line so multi-byte
chars split across chunks survive, caps the buffer at 1 MiB
(SSE_EVENT_BUFFER_CAP) and dispatches a pending event at EOF.
forward_stream threads the parser through its unfold state and emits a
terminal error envelope on cap overflow. Existing single-chunk SSE test
assertions preserved; added multi-chunk, split-UTF-8, EOF-dispatch, and
cap tests.
Verified: cargo test (219 pass), clippy -D warnings, fmt --check.
glm-5.3-flash
cc34c08e4e