[MINOR][INFRA] Ignore AGENTS.md and CONTRIBUTING.md in determine_modules_for_files

### What changes were proposed in this pull request?

This PR extends `determine_modules_for_files` in `dev/sparktestsupport/utils.py` to
ignore `AGENTS.md` and `CONTRIBUTING.md` in addition to the existing `README.md`.

### Why are the changes needed?

A documentation-only PR that touches only `AGENTS.md` (e.g.
#55707) currently triggers all CI test jobs because
the file is not associated with any submodule, so it falls through to the `root`
module. Neither file affects code or tests, and neither is consumed by the docs build,
so they should be ignored just like `README.md`.

### Does this PR introduce _any_ user-facing change?

No, this is only a testing infra change.

### How was this patch tested?

Updated and ran the doctests in `dev/sparktestsupport/utils.py`.

### Was this patch authored or co-authored using generative AI tooling?

Generated-by: Claude Opus 4.7

Author: zhengruifeng

dev/sparktestsupport/utils.py

@@ -34,18 +34,22 @@ def determine_modules_for_files(filenames):
     Given a list of filenames, return the set of modules that contain those files.
     If a file is not associated with a more specific submodule, then this method will consider that
     file to belong to the 'root' module. `.github` directory is counted only in GitHub Actions,
-    and `README.md` is always ignored.
+    and `README.md`, `AGENTS.md`, `CONTRIBUTING.md` are always ignored.
 
     >>> sorted(x.name for x in determine_modules_for_files(["python/pyspark/a.py", "sql/core/foo"]))
     ['pyspark-core', 'pyspark-errors', 'sql']
     >>> [x.name for x in determine_modules_for_files(["file_not_matched_by_any_subproject"])]
     ['root']
     >>> [x.name for x in determine_modules_for_files(["sql/README.md"])]
     []
+    >>> [x.name for x in determine_modules_for_files(["AGENTS.md"])]
+    []
+    >>> [x.name for x in determine_modules_for_files(["CONTRIBUTING.md"])]
+    []
     """
     changed_modules = set()
     for filename in filenames:
-        if filename.endswith("README.md"):
+        if filename.endswith(("README.md", "AGENTS.md", "CONTRIBUTING.md")):
             continue
         if filename in (
             "scalastyle-config.xml",