fix: strip DeepSeek DSML tool-call markers from assistant content

[sharadvc]

Problem

DeepSeek's DSML protocol injects tool-call routing markers into the content stream before the actual tool_calls delta arrives. These markers (e.g. <‖DSML‖tool_calls>, <‖DSML‖invoke ...>) leak into the UI when tool calls fail or aren't parsed in time.

Fix

Adds _strip_dsml_markers() — a fast regex filter applied to every content chunk yielded by _parse_openai_sse().

• Zero overhead on non-DeepSeek streams: the guard short-circuits on chunks that don't contain 'DSML'
• Catches all known marker variants: full-width ‖, Unicode ｜, ASCII pipe |, and double-escaped forms
• Applied at both the streaming yield point (real-time UI) and the final block assembly (fallback)

Testing

• Verified with synthetic SSE chunks containing <‖DSML‖tool_calls>, <‖DSML‖invoke name="ShellCommand" arguments="{...}"> — all stripped cleanly
• Non-DeepSeek streams unaffected (guard short-circuits)

Closes #804