models: Clamp demo redacted values to parseable constants

- Updated `_demo_value_sanitizers.py` to enforce parseable-but-useless
  replacements for demo-redacted boolean-like strings to maintain operability
  while preserving data obscurity.
- Added comments clarifying the intent of replacements in demo mode.
- Updated AGENTS.md to document the redaction approach for demo/free-tier
  values.

Assisted-by: Codex

Author: datalogics-kam

AGENTS.md

@@ -173,6 +173,10 @@
   pdfRest wire quirks (for example, splitting comma-separated values or
   serializing only the first uploaded file ID), not re-implement constraint
   logic already expressed by Pydantic field types/annotations.
+- For demo/free-tier redactions, favor parseable-but-useless replacements over
+  reconstructing likely true values. The SDK should remain operable (no parsing
+  crashes) while preserving demo mode’s intent of withholding useful output
+  fidelity.
 - Prefer reusable validator factories that take parameters (for example
   allowed-value/extension helpers with keyword-configured fallbacks) over
   bespoke one-off validator functions tied to a single field.

src/pdfrest/models/_demo_value_sanitizers.py

@@ -53,6 +53,9 @@ def _demo_bool_or_passthrough(
     if value is None or isinstance(value, bool):
         return value
     if _looks_like_demo_redaction(value):
+        # Intentionally clamp demo-redacted bool-like strings to a configured
+        # constant. The goal is parseability without restoring potentially
+        # meaningful signal that demo mode is designed to obscure.
         _log_replacement(value, replacement, info)
         return replacement
     return value