fix(core): allow thought-only responses in GeminiChat stream validation (#3251)

Models using thinking/reasoning modes may emit only thought content
without explicit text output. The stream validation previously rejected
these as 'empty' responses. Now accepts responses that contain either
text content or thought content when a finish reason is present.

(cherry picked from commit a0b13911f41d2d1878bb12999e4794785a8f75e3)

Co-authored-by: mingholy.lmh <mingholy.lmh@alibaba-inc.com>