Add an optional `header` parameter to `FeishuStreamingSession.start()`
so that streaming cards can display a colored title bar, matching the
appearance of non-streaming interactive cards.
The Card Kit API already supports `header` alongside `streaming_mode`,
but the current implementation omits it, producing headerless cards.
This change is fully backward-compatible: when `header` is not provided,
behavior is identical to before.
Closes#13267 (partial)
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
8a2273e210