feat(utils): add Jinja2-based instruction templating with use_jinja2 flag

[guptaishaan]

The existing regex-based approach in inject_session_state does not support
control-flow constructs such as conditionals and loops. This change introduces
optional Jinja2 rendering as a first-class alternative.

What changed

• The original substitution logic is extracted into a private helper
  _render_with_regex — behaviour is identical, no breaking changes.
• A new private helper _render_with_jinja2 is added. It creates an
  async-capable jinja2.Environment, exposes all session state keys as
  template variables, and injects an async artifact(filename) callable so
  templates can load artifacts inline (e.g. {{ artifact('doc.txt') }}).
• inject_session_state gains an optional use_jinja2: bool = False keyword
  argument. When True it delegates to _render_with_jinja2; when False
  (the default) it calls _render_with_regex, preserving full backward
  compatibility.
• Unit tests are added for basic variable substitution, conditional blocks,
  for-loops, artifact loading, and the missing-artifact error path, all via
  use_jinja2=True.

Jinja2 is already a declared dependency of the package so no new requirements
are introduced.

Fixes #2942