fix(example): derive streaming response parser boundaries from schema

[abetlen]

Fix streaming response parsing for schema-defined chat markers.

• Derives assistant prefixes, reasoning spans, and stop markers from response_schema regex structure instead of hardcoded conversation tags.
• Keeps terminal streaming chunks on the incremental parser path so incomplete reasoning spans do not leak raw markers.
• Validated with parser smoke checks plus Qwen and Gemma endpoint smoke tests for chat, reasoning, tool calls, and Responses streaming.