#2700 serve: preserve code-quoted output markers

说明

# Issue: Code-quoted protocol markers alter streamed channels or tool calls

Follow up #537. Markdown-quoted thinking/call markers can be interpreted as
protocol. Unfinished quotes can hide an answer; forced termination can release
a speculative tool call.

# PR: Preserve code-quoted markers across stream boundaries

Share incremental Markdown context between thinking and tool parsing. Resolve
held quotes at genuine boundaries and distinguish forced closes/termination.
Keep upstream single/double-quote handling and bound scanning work.

Validation: `serve.test_code_span_quotes`, `serve.test_parser_track`; exact channel
text, delimiter splits, escapes, fences, incomplete quotes, genuine calls,
forced termination, both APIs and scan-work bounds.