…eholders (unslothai#5763)

* fix(chat_templates): check find() return value before slicing on placeholders

Two places in `construct_chat_template()` use `str.find()` for sentinel
placeholders (`{INPUT}` / `{OUTPUT}`) without checking the -1 return:

1. The `except:` fallback (around line 2464) computes
   `chat_template[chat_template.find("{OUTPUT}") + len("{OUTPUT}"):]`.
   If the template has no `{OUTPUT}` marker, `find()` returns -1 and the
   slice starts at offset 7 (`-1 + len("{OUTPUT}")`), producing garbage
   that's then `re.escape`-d and fed back into the template-recovery
   regex. The user sees a confusing `IndexError` on
   `response_part = response_part[0]` instead of the real problem.

2. The final trim before returning (`input_part[:input_part.find("{INPUT}")]`
   and the matching `{OUTPUT}` line) silently drops the last character
   when the placeholder is missing — `find()` returns -1, and `[:-1]`
   slices everything except the last character, returning a corrupted
   template prefix to the caller.

Replace both with an explicit `-1` check that raises a clear
`RuntimeError` naming the missing placeholder, matching the existing
guard pattern from unslothai#4923 (`try_fix_tokenizer`).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

* fix(chat_templates): also guard {INPUT} and fallback regex/separator paths

Builds on the {OUTPUT} / final-trim guards in this branch by closing
the three remaining ways the except-block fallback in
construct_chat_template() can still raise a confusing IndexError or
AttributeError on malformed templates:

1. Validate both {INPUT} and {OUTPUT} before deriving `ending`. The
   regex two lines later (`{INPUT} + ending + ...`) still produced an
   empty list and crashed on `response_part[0]` if {INPUT} was missing.
2. Guard the regex no-match case. Some templates contain both
   placeholders but not in a recoverable two-example shape, in which
   case `re.findall` returns an empty list and `[0]` raises.
3. Initialize `found = None` before the separator-search loop and
   raise if the loop never sets it. Previously, if the first
   iteration's `re.finditer` was empty the loop broke without binding
   `found`, and `found.group(1)` raised AttributeError on the stale
   int left over from the outer rfind loop.

Rephrase the final-trim error messages from internal variable names
("input_part") to user-facing wording ("instruction section") and
include a bounded (200-char) excerpt of the offending content so the
error is debuggable without being unbounded.

Add tests/python/test_construct_chat_template_validation.py covering
each failure mode with a fake tokenizer (no HF_TOKEN, no model
download, CPU-only).

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>
Co-authored-by: Daniel Han <danielhanchen@gmail.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

…eholders (unslothai#5763)

* fix(chat_templates): check find() return value before slicing on placeholders

Two places in `construct_chat_template()` use `str.find()` for sentinel
placeholders (`{INPUT}` / `{OUTPUT}`) without checking the -1 return:

1. The `except:` fallback (around line 2464) computes
   `chat_template[chat_template.find("{OUTPUT}") + len("{OUTPUT}"):]`.
   If the template has no `{OUTPUT}` marker, `find()` returns -1 and the
   slice starts at offset 7 (`-1 + len("{OUTPUT}")`), producing garbage
   that's then `re.escape`-d and fed back into the template-recovery
   regex. The user sees a confusing `IndexError` on
   `response_part = response_part[0]` instead of the real problem.

2. The final trim before returning (`input_part[:input_part.find("{INPUT}")]`
   and the matching `{OUTPUT}` line) silently drops the last character
   when the placeholder is missing — `find()` returns -1, and `[:-1]`
   slices everything except the last character, returning a corrupted
   template prefix to the caller.

Replace both with an explicit `-1` check that raises a clear
`RuntimeError` naming the missing placeholder, matching the existing
guard pattern from unslothai#4923 (`try_fix_tokenizer`).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

* fix(chat_templates): also guard {INPUT} and fallback regex/separator paths

Builds on the {OUTPUT} / final-trim guards in this branch by closing
the three remaining ways the except-block fallback in
construct_chat_template() can still raise a confusing IndexError or
AttributeError on malformed templates:

1. Validate both {INPUT} and {OUTPUT} before deriving `ending`. The
   regex two lines later (`{INPUT} + ending + ...`) still produced an
   empty list and crashed on `response_part[0]` if {INPUT} was missing.
2. Guard the regex no-match case. Some templates contain both
   placeholders but not in a recoverable two-example shape, in which
   case `re.findall` returns an empty list and `[0]` raises.
3. Initialize `found = None` before the separator-search loop and
   raise if the loop never sets it. Previously, if the first
   iteration's `re.finditer` was empty the loop broke without binding
   `found`, and `found.group(1)` raised AttributeError on the stale
   int left over from the outer rfind loop.

Rephrase the final-trim error messages from internal variable names
("input_part") to user-facing wording ("instruction section") and
include a bounded (200-char) excerpt of the offending content so the
error is debuggable without being unbounded.

Add tests/python/test_construct_chat_template_validation.py covering
each failure mode with a fake tokenizer (no HF_TOKEN, no model
download, CPU-only).

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>
Co-authored-by: Daniel Han <danielhanchen@gmail.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>