build(deps): bump dtschema from 2025.12 to 2026.4

Bumps [dtschema](https://github.com/devicetree-org/dt-schema) from 2025.12 to 2026.4.
- [Release notes](https://github.com/devicetree-org/dt-schema/releases)
- [Commits](devicetree-org/dt-schema@v2025.12...v2026.04)

---
updated-dependencies:
- dependency-name: dtschema
  dependency-version: '2026.4'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

requirements.txt

@@ -16,5 +16,5 @@ PyYAML == 6.0.3        # for parsing/writing YAML
 oras == 0.2.42         # for OCI stuff in mapper-oci-update
 Jinja2 == 3.1.6        # for templating
 rich == 14.3.3         # for rich text formatting
-dtschema == 2025.12     # for checking dts files and dt bindings
+dtschema == 2026.4     # for checking dts files and dt bindings
 yamllint == 1.38.0     # for checking dts files and dt bindings