fix(sdk): use build_replacements unresolved set instead of post-scan for curly templates

The curly template formatter raised a false TemplateFormatError when a
substituted value happened to contain {{...}} patterns (e.g. template
examples or code snippets in user input).

Root cause: after substitution, compute_truly_unreplaced() re-scanned
the rendered output for {{...}} patterns and intersected with the
original placeholder set. It could not distinguish between genuinely
unreplaced placeholders and {{...}} patterns injected by substituted
values.

Fix: use the 'unresolved' set already returned by build_replacements()
to detect truly missing variables. This set is computed before
substitution, so it is immune to patterns in substituted values.

Changes:
- types.py: use unresolved from build_replacements instead of
  compute_truly_unreplaced
- handlers.py: same fix
- Remove now-unused compute_truly_unreplaced from both files
- Add unit tests covering the false-positive scenario

Fixes #3770

Author: stakeswky