feat: implement brand override backend logic

[rbcorrales]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Add support for overriding the current brand via URL parameter (?brand=brand-slug) and filters.

• Add brand override check to the taxonomy class.
• Update brand resolution methods to accept optional brand override parameter.
• Add newspack_brand_override filter for custom override logic.
• Add extensive test coverage for brand override behavior across posts, author archives, term archives, brand archives, homepage and filter functionality.

Rules:

The brand override system follows these rules:

1. Posts:

• If a post has a single brand, that brand is always used and overrides are ignored.
• If a post has multiple brands:
  • If a valid override (via URL or filter) matches one of the post brands, it is used.
  • If no valid override is present but a primary brand is set, the primary brand is used.
  • If neither a valid override nor a primary brand is present, the post is neutral (no brand).

2. Author archives:

• If the author has a valid primary brand, that brand is always used and overrides are ignored.
• If the author has no primary brand, a valid override is used if present.
• If neither a primary brand nor a valid override is present, the page is neutral.

3. Term archives (categories/tags):

• If the term or any ancestor has a primary brand, that brand is used and overrides are ignored.
• If no primary brand is found in the term or ancestors, a valid override is used if present.
• If neither a primary brand nor a valid override is present, the page is neutral.

4. Brand archives ignore overrides.
5. Homepage ignores overrides.

How to test the changes in this Pull Request:

• Posts (Single Brand)

  • Create a post (or edit an existing one) and assign it a single brand (e.g. brand‑a).
  • Visit the post's permalink (e.g. /my‑post) and verify that the post's brand (brand‑a) is used.
  • Then, append ?brand=brand‑b (or any other brand slug) to the URL and reload.
  • Verify that the post's brand (brand‑a) is still used (i.e. the override is ignored).

• Posts (Multiple Brands)

  • Create (or edit) a post and assign it multiple brands (e.g. brand‑a and brand‑b). Optionally, set a primary brand.
  • Visit the post's permalink (e.g. /my‑multi‑brand‑post) and verify that (if a primary brand is set) that brand is used.
  • Append ?brand=brand‑a (or ?brand=brand‑b) and reload.
  • Verify that the override is used (if it's one of the post's brands).
  • Append ?brand=non‑existent‑brand and reload. Verify that if a primary brand is set the primary brand is used. Otherwise, the post should be neutral.

• Author Archives

  • If the author has a primary brand set, visit the author's archive (e.g. /author/author‑name) and verify that the author's primary brand is used.
  • Append ?brand=brand‑a or any other brand slug that is not the primary for the author and reload.
  • Verify that the author's primary brand is still used (the override is ignored).
  • If the author does not have a primary brand, visit the author's archive and append ?brand=brand‑a or any other valid brand slug.
  • Verify that the override (brand‑a) is used.

• Term Archives (Categories/Tags)

  • If your term (or an ancestor) has a primary brand set, visit the term's archive (e.g. /category/my‑cat) and verify that the primary brand (or ancestor's primary brand) is used.
  • Append ?brand=brand‑a or any other brand slug that is not the primary for the category and reload.
  • Verify that the primary brand (or ancestor's primary) is still used (the override is ignored).
  • If your term (and its ancestors) do not have a primary brand, visit the term's archive and append ?brand=brand‑a or any other valid brand slug.
  • Verify that the override (brand‑a) is used.

• Brand Archives

  • Visit a brand archive (e.g. /brand‑a) and verify that the brand (brand‑a) is used.
  • Append ?brand=brand‑b (or any other brand slug) and reload.
  • Verify that the brand (brand‑a) is still used (the override is ignored).

• Homepage

  • Visit the homepage (e.g. /) and verify that the page is neutral (no brand is used).
  • Append ?brand=brand‑a (or any other brand slug) and reload.
  • Verify that the homepage remains neutral (i.e. the override is ignored).

• Custom Override (Filter)

  • Add a custom filter that hooks into the newspack_brand_override and verify that the filter's logic (for example, overriding with a different brand or returning null) is applied as expected.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?