[autosummary] fix suffix detection

fix sphinx-doc#12147

`SphinxComponentRegistry.source_parsers` uses `rst`, `md` and so
on (without `.`) as key but `source_suffix` in `conf.py` uses `.rst`,
`md` and so on (with `.`) as key. So we need to remove preceding `.`
from suffixes from `source_suffix` to detect supported format from
`SphinxComponentRegistry.source_parsers`.

CHANGES.rst

@@ -127,6 +127,8 @@ Bugs fixed
   Patch by James Addison.
 * #11578: HTML Search: Order non-main index entries after other results.
   Patch by Brad King.
+* #12147: autosummary: Fix a bug that wrong suffix may be used with multiple ``source_suffix``.
+  Patch by Sutou Kouhei.
 
 Testing
 -------

sphinx/ext/autosummary/__init__.py

@@ -770,7 +770,7 @@ def run(self) -> tuple[list[Node], list[system_message]]:
 
 def get_rst_suffix(app: Sphinx) -> str | None:
     def get_supported_format(suffix: str) -> tuple[str, ...]:
-        parser_class = app.registry.get_source_parsers().get(suffix)
+        parser_class = app.registry.get_source_parsers().get(suffix.removeprefix('.'))
         if parser_class is None:
             return ('restructuredtext',)
         return parser_class.supported