fix(agent): ignore quoted scratchpad tags in retry detection

[sgaofen]

Summary

• stop has_incomplete_scratchpad() from treating quoted or code-form scratchpad tag mentions as an unfinished reasoning block
• strip fenced code blocks, inline code, and blockquotes before checking for unclosed <REASONING_SCRATCHPAD> tags
• add focused regression coverage for real incomplete tags plus the false-positive markdown cases

Root Cause

The retry guard in agent/trajectory.py used a plain substring check for <REASONING_SCRATCHPAD> without understanding markdown context. That meant grep output, quoted user text, or inline code containing the tag could trigger the incomplete-scratchpad path, causing wasted retries and dropped output.

Closes #11663.

Validation

• python3 -m py_compile agent/trajectory.py tests/agent/test_trajectory.py
• uv run --extra dev pytest -o addopts='' tests/agent/test_trajectory.py -q

Platform Tested

• macOS 15.x (Apple Silicon)

Contribution Guide Notes

• Reviewed CONTRIBUTING.md and checked for existing open PRs before submitting this scoped change.
• Ran the targeted verification commands listed above for this PR. I have not claimed a full repo-wide pytest tests/ -q pass unless explicitly noted.

[trevorgordon981]

LGTM. Switching from presence to count-based comparison and stripping markdown context before checking is the right approach. Tests cover the key false-positive vectors (fenced, inline, blockquote) and the "real tag after quote" case pins the true-positive path.

One minor edge case to be aware of (not blocking): _BLOCKQUOTE_RE only matches single blockquote lines. A multi-line continuation blockquote like:

> first line
> <REASONING_SCRATCHPAD>

is handled correctly since each > line matches independently. But a bare continuation-style blockquote without repeated > markers on subsequent lines would not be stripped. In practice this probably never fires since models/users don't tend to produce bare continuation blockquotes containing scratchpad tags.