Add unit test coverage for shared pure-logic modules (240 tests)

Track 1.1 of master plan: expand from sexp-only tests to cover
DTOs, HTTP signatures, HMAC auth, URL utilities, Jinja filters,
calendar helpers, config freeze, activity bus registry, parse
utilities, sexp helpers, error classes, and jinja bridge render API.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/tests/conftest.py

@@ -0,0 +1,10 @@
+"""Shared test fixtures for unit tests."""
+from __future__ import annotations
+
+import os
+import sys
+
+# Ensure project root is on sys.path so shared.* imports work
+_project_root = os.path.dirname(os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+if _project_root not in sys.path:
+    sys.path.insert(0, _project_root)