Removal of parser.set_application breaks parser

### Describe the bug PR #13644 and PR #13637 break my bibtex parser, as uses the `set_application` hook to get config values. What is the preferred way of getting sphinx config values an extension registers, from a parser? The [docs](https://www.sphinx-doc.org/en/master/extdev/parserapi.html) still say to use `set_application`, `.config` and `.env`, all of which are deprecated. ### How to Reproduce Minimal reproduction would be to use a parser that accesses `set_application`, like [sphinx_bib_domain](https://github.com/jgrey4296/sphinx_bib_domain) and try to parse a `.bib` file. ### Environment Information ```text Platform: linux; (Linux-6.17.0-20-generic-x86_64-with-glibc2.39) Python version: 3.12.3 (main, Mar 3 2026, 12:15:18) [GCC 13.3.0]) Python implementation: CPython Sphinx version: 9.1.0 Docutils version: 0.21.2 Jinja2 version: 3.1.6 Pygments version: 2.19.2 ``` ### Sphinx extensions ```python ["sphinx_bib_domain"] ``` ### Additional context _No response_