Require reasoning tags to start at line beginning

[jannikstdl]

Motivation

• Reduce false-positive detection of reasoning tags when they appear inline, in prose, or as printed examples so tag parsing only triggers on intended model streaming markers.
• Preserve previous protection against tags appearing inside fenced code blocks to avoid breaking code samples.
• Improve robustness of the streaming parser so examples like printed <think>...</think> do not change parsing state.

Description

• Updated backend/open_webui/utils/middleware.py to add is_tag_at_line_start and require tags to start at the beginning of a line (ignoring leading whitespace).
• Continued to skip matches inside fenced code blocks using the existing is_inside_fenced_code_block helper.
• Replaced single re.search with iterating re.finditer so candidate matches can be validated with both fenced-code and line-start checks before accepting.
• Preserved existing attribute extraction and downstream tag_content_handler behavior when a valid start tag is found.

Testing

• No automated tests were executed for this change.
• Changes were added and committed to backend/open_webui/utils/middleware.py without running test suites.

---

Codex Task