Require handlebars for template field validation

[alexmojaki]

Summary

• remove the Python 3.9-era no-op path from template field validation
• require the handlebars-backed validator to run when collecting template field issues
• remove stale skip decorators from the template field validation push tests

Tests

• uv run pytest tests/test_push_variables.py -q
• uv run ruff check logfire/variables/_handlebars.py logfire/variables/abstract.py tests/test_push_variables.py
• uv run pyright logfire/variables/_handlebars.py logfire/variables/abstract.py tests/test_push_variables.py

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 1 additional finding.

[cubic-dev-ai]

No issues found across 3 files

Confidence score: 5/5

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

_{Re-trigger cubic}