fix(streaming): scrub leaked partial tool-call opener fragments per-delta

[davidgut1982]

Summary

Follow-up to #14251 (strip standalone tool-call XML from visible text), which
handled the post-streaming final-text path but explicitly deferred the live
per-delta scrub. This adds that per-delta scrub so partial tool-call opener
fragments never reach user-facing streamed output.

The bug / how it manifests

Qwen3-class models sometimes begin emitting an XML tool-call opener as ordinary
content (<tool_call>) and then switch to native tool_calls mid-token,
leaving a stray content delta such as "ool_call>" that streams straight to
user-facing output. These tails are never valid prose.

The fix

• utils.py: add strip_partial_toolcall_fragments(), using a single compiled
  regex for both search and substitution (no search/sub asymmetry) that matches
  the leaked tails of <tool_call> / <function_call> openers.
• agent/chat_completion_helpers.py: wire it into the stream_delta_callback
  branch of interruptible_streaming_api_call. That branch bypasses
  _fire_stream_delta and therefore did not previously get the scrub. The
  callback and text-record now guard on the scrubbed result, so a pure-fragment
  delta fires no callback and records no text.

Notes

• Directly continues the deferred follow-up called out in fix(display): strip standalone tool-call XML tags from visible text (port openclaw#67318) #14251.
• No behaviour change for well-formed deltas: only leaked partial-opener tails
  are removed.

🤖 Generated with Claude Code

[alt-glitch]

This was generated by AI during triage.

Duplicate of #33355 — same new strip_partial_toolcall_fragments() helper in utils.py and the same wiring into the stream_delta_callback branch of chat_completion_helpers.py, both as the deferred per-delta follow-up to #14251. #33355 is the earlier open PR and is broader: it also scrubs the primary run_agent.py::_fire_stream_delta (Path A) content path, whereas this PR only covers the chat_completion_helpers.py branch (Path B). Related: #46330 (Harmony streaming scrubber, adjacent family).