Remove Python 3.9 handlebars test skips

[alexmojaki]

Summary

• remove Python 3.9-era pydantic-handlebars skip wrappers from the managed variable tests
• remove Python-version-specific docs and optional dependency error wording for template variables
• keep the optional-extra error path because pydantic-handlebars is still provided by logfire[variables], not the core install

Tests

• uv run pytest tests/test_variable_templates.py tests/test_template_validation.py tests/test_variable_composition.py tests/test_docs.py -q
• uv run ruff check logfire/variables/_handlebars.py tests/test_docs.py tests/test_template_validation.py tests/test_variable_composition.py tests/test_variable_templates.py
• uv run pyright logfire/variables/_handlebars.py tests/test_docs.py tests/test_template_validation.py tests/test_variable_composition.py tests/test_variable_templates.py

[devin-ai-integration]

✅ Devin Review: No Issues Found

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

[cubic-dev-ai]

No issues found across 7 files

Confidence score: 5/5

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

_{Re-trigger cubic}