The lazy `<think>.*?</think>` pattern (one compiled `_THINK_RE`, one inline
copy) is applied with `re.sub` over whole model responses. With a `<think>`
opener and no closer, the engine rescans to end-of-string from every opener
-> O(n^2) on attacker-influenced output (prompt injection can echo thousands
of openers via tool output / retrieved content). CodeQL py/polynomial-redos.
Replace both with `_strip_think_blocks`, a forward-only linear scan that is
byte-for-byte equivalent to the original narrow regex: only literal
`<think>`/`</think>` (any case) match, a dangling opener with no closer is
left intact, and an orphan `</think>` is never stripped. Routing through the
broader `text_helpers.strip_think` was avoided on purpose -- it also strips
`<thinking>`, attributes and prompt echoes, which would change what the
loop's progress/circling heuristics see.
Adds tests/test_redos_think_blocks.py pinning regex-equivalence on a battery
of well-formed/edge inputs plus a linear-time bound on hostile input.
nopoz