Bump pydantic-handlebars to 0.2.1 and drop escape-coupling shims

[dmontagu]

Stacked on #1962. Bumps the dep and cleans up the now-stale escape shims that #1962 left a coupling comment about.

Why now

pydantic-handlebars 0.2.1 ships spec-compliant escape semantics: a run of N backslashes preceding an open delimiter contributes N // 2 literal \ characters, with parity deciding whether the mustache renders (even) or stays literal (odd).

That makes the logfire-side _HAS_REFERENCE regex ((?<!\\)@\{ — "any preceding backslash means escaped") actively wrong for even-backslash cases. With 0.2.1:

Source	Old logfire behaviour	Correct (post-0.2.1)
\@{x}@	literal @{x}@	literal @{x}@ ✓
\\@{x}@	literal \@{x}@	\X
\\\@{x}@	literal \\@{x}@	literal \@{x}@ ✓
\\\\@{x}@	literal \\\@{x}@	\\X

The regex returned has_references == False for any \@{ prefix, so even-backslash cases short-circuited the composition path and emerged unrendered. The fix routes everything containing @{ through the renderer, which now handles parity correctly.

What changed

• pyproject.toml: pydantic-handlebars>=0.2.0 → >=0.2.1 in both the [variables] extra and the dev group.
• composition.py cleanup:
  • has_references drops the regex; becomes '@{' in serialized_value. Doing parity-correct detection in pure regex would need a variable-width lookbehind we can't write portably in re on 3.10/3.11; with the renderer as the source of truth it isn't needed.
  • _render_value no longer does the manual \@{ → @{ substitution for "no refs" strings. Strings with no @{ pass through unchanged; strings with @{ route through the renderer.
  • expand_references drops the special-case early return that called _unescape_serialized when no real refs resolved. _render_value handles escape-only strings on its own walk.
  • _unescape_serialized is unused and deleted.
  • _HAS_REFERENCE regex deleted (with the coupling-note comment).

Tests

Two pre-existing tests asserted the old (buggy by spec) behaviour and have been updated to assert the correct behaviour:

• test_keyword_block_references_are_ignored — @{#if this}@yes@{/if}@ now renders yes (real Handlebars if block against the empty context, which is truthy). The composed == [] assertion still pins the property that this and if aren't resolved as variable lookups.
• test_referenced_escaped_reference_is_preserved — \@{not_a_ref}@ now renders as @{not_a_ref}@ (escape consumed per spec).

New test test_backslash_run_parity_under_composition pins the parity table (N = 1, 2, 3, 4) end-to-end through Variable.get() so the previously-buggy even-backslash cases stay correct.

447 variable / composition / template / push tests pass. Pyright + ruff format + check clean.

Merge ordering

Targets feature/variable-composition-tier1-fixes (#1962's branch). Once #1962 merges → #1954 → main, this rebases onto main.

[cubic-dev-ai]

No issues found across 4 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Re-trigger cubic}