fix: don't discard model output past a literal reasoning close tag

[nolanchic]

Problem

For models with a reasoning_tag (e.g. DeepSeek-R1 → think), remove_reasoning_content silently discarded part of the model's real answer whenever the answer contained a literal closing tag (e.g. </think> inside code, markup, or prose) and the response also contained a real reasoning block.

What happens: the function first strips balanced <think>...</think> blocks, then has a fallback that drops everything before any remaining </think> (intended for the continuation case where the opening tag was emitted in a prior chunk). That fallback fired unconditionally, so a literal </think> in the answer was mistaken for an orphan reasoning close.

in:  <think>r</think>Here is code: print('</think>')
was: ')
now: Here is code: print('</think>')

This function runs on every assistant response for reasoning-tag models (Coder.remove_reasoning_content) and on every commit-message / chat-summarization call (Model.simple_send_with_retries), so the truncation was silent and the shortened text was what reached edit parsing / commit-message generation.

Fix

aider/reasoning_tags.py — only treat a remaining </tag> as an orphan close when the response contained no opening tag at all (the continuation case). When an opening tag was present, any </tag> left after the balanced-block substitution is a literal occurrence in the answer and is preserved.

The legitimate orphan-close behaviour (continuation whose <tag> was in a prior chunk) is unchanged.

Test plan

• Added test_remove_reasoning_content_literal_closing_tag — reproduces the truncation on main (returns ')'), passes with the fix.
• Added test_remove_reasoning_content_orphan_closing_tag_stripped — locks in that a genuine orphan close (no opening tag) is still stripped.
• pytest tests/basic/test_reasoning.py tests/basic/test_models.py — 34 passed (incl. the existing test_remove_reasoning_content and the integration tests that drive coder.remove_reasoning_content() with mocked streaming).
• pre-commit run --files aider/reasoning_tags.py tests/basic/test_reasoning.py — isort / black / flake8 / codespell all pass.