Add named template partials to the DTL.

[Carlton Gibson]

Completing the cycle begun in 2023, incorporate named template partials into the Django Template Language.

Starting with the forum discussion on ​Adding template fragments or partials for the DTL we added the internal structures necessary for template partials in #34883.

The third-party ​django-template-partials package, first released during the Django 4.2 cycle, has battle-tested the implementation over a full LTS cycle, and gained significant community uptake.

The project to merge this into Django was accepted for GSoC 2025, with Farhan Ali Raza implementing it.

The project is to merge to the two template tags, for defining and rendering a partial, plus the integration with template loader, so that partial fragments can be loaded by name. A migration guide will be added to the django-template-partials repo for existing users.

[Farhan Ali]

There are a couple of comments remaining about code moving. But it is not clear what decision is made about those.

[nessita <124304+nessita@…>]

In 5e06b970:

Fixed #36410 -- Added support for Template Partials to the Django Template Language.

Introduced {% partialdef %} and {% partial %} template tags to
define and render reusable named fragments within a template file.
Partials can also be accessed using the template_name#partial_name
syntax via get_template(), render(), {% include %}, and other
template-loading tools.

Adjusted get_template() behavior to support partial resolution, with
appropriate error handling for invalid names and edge cases. Introduced
PartialTemplate to encapsulate partial rendering behavior.

Includes tests and internal refactors to support partial context
binding, exception reporting, and tag validation.

Co-authored-by: Carlton Gibson <carlton@…>
Co-authored-by: Natalia <124304+nessita@…>
Co-authored-by: Nick Pope <nick@…>

[GitHub <noreply@…>]

In 892a45d:

Refs #36410 -- Added link to migration guide for template-partials.

[Jacob Walls <jacobtylerwalls@…>]

In 0e0b4214:

Fixed #36589 -- Made assertTemplateUsed/NotUsed track full path for PartialTemplate.

Previously, assertTemplateUsed only matched partial names, ignoring
the template origin. This caused assertions on partials specified by
origin ("template.html#partial") to fail. Refs #36410.