test: lock preprocessor boundary fixtures

[rlippmann]

What changed

• Added conservative preprocessor guard fixtures for abstain-only boundary cases (multiline, sentence-adjacent, fenced code block, inline prose/code, reported speech, mixed directive+task, conversational question, nested wrappers).
• Added positive fixtures for existing safe canonicalizations (case normalization, whitespace collapse).
• Updated preprocessor docs to explicitly define whole-message canonicalization boundaries and abstain-first policy.

Why

• Lock expected 0.7 conservative preprocessor behavior with explicit fixture coverage.
• Prevent boundary drift by documenting and testing abstain behavior for directive-like text embedded in prose/mixed-intent formats.
• Keep preprocessor scope narrow (format canonicalization only) without expanding natural-language parsing semantics.
• This resolves the heuristic boundary portion of Preprocessor behavior for multi-directive input (clarify vs abstain) #77 by fixture-locking conservative abstain behavior; fallback-boundary hardening remains follow-up work.

Refs #77

Checklist

• pre-commit run (uv run pre-commit run --all-files)
• tests pass (uv run pytest)