Fix pre-commit failures: ruff import spacing + SPDX sidecars

pre-commit.ci on #843 surfaced two issues:
- ruff I001: generate.py was missing the blank line required by
  the repo's lines-after-imports = 2 isort setting.
- reuse lint: extra.txt had no copyright/licensing information.
  Adding sidecar .license files is the repo's established pattern
  for plain-text fixtures (see tests/packages/install-data/data.txt.license).

Also add sidecars for the two static.cdx.json fixtures for good measure
-- CI didn't flag them but local reuse runs do. Both are one-line
sidecars matching the rest of the repo's style.

Author: fangchenli

tests/packages/dist-info-sboms-platlib/sboms/static.cdx.json.license

@@ -0,0 +1,3 @@
+# SPDX-FileCopyrightText: 2026 The meson-python developers
+#
+# SPDX-License-Identifier: MIT

tests/packages/dist-info-sboms/generate.py

@@ -7,6 +7,7 @@
 import json
 import sys
 
+
 with open(sys.argv[1], 'w') as f:
     json.dump({
         'bomFormat': 'CycloneDX',

tests/packages/dist-info-sboms/sboms/extra.txt.license

@@ -0,0 +1,3 @@
+# SPDX-FileCopyrightText: 2026 The meson-python developers
+#
+# SPDX-License-Identifier: MIT

tests/packages/dist-info-sboms/sboms/static.cdx.json.license

@@ -0,0 +1,3 @@
+# SPDX-FileCopyrightText: 2026 The meson-python developers
+#
+# SPDX-License-Identifier: MIT