Tags: add `BaseTag.from_jinja()` to convert `Jinja` output into real Air Tags via `from_html`

[pygarap]

Summary

Add a strict BaseTag.from_jinja() helper that renders a Jinja template to HTML and immediately converts it into real Air Tags nodes via BaseTag.from_html.

This should be strict-only: invalid HTML must raise, matching BaseTag.from_html behavior.

Motivation

Mixing Air Tags and Jinja is a valid use case, but embedding rendered Jinja output as a raw HTML string (SafeStr / trusted HTML) inside an Air Tags tree makes that subtree opaque to anything that expects real tags (tree transforms, structure-based features, future tooling, etc.).

We already have a non-strict path that returns template output as a string for insertion into tags. That was implemented in PR #981 and closed issue #231. This request is specifically for the strict option that converts template output into actual Air Tags nodes.

Proposed API

BaseTag.from_jinja(template, /, **context) -> BaseTag

Behavior:

1. Render the Jinja template to an HTML string.
2. Parse using BaseTag.from_html(rendered_html).
3. Return the resulting Air Tags tree.

Notes:

• No non-strict mode here. If parsing fails, raise (same as BaseTag.from_html).

References

• Existing non-strict behavior: PR Make Jinja render from within air tags #981
• Related issue (already closed by the PR above): Jinja renderer not rendering from within Air Tags #231

Acceptance criteria

• BaseTag.from_jinja() exists and is documented.
• Tests cover:
  • valid fragment output
  • full document output
  • invalid HTML raises (no fallback)