chore(deps): bump actions/checkout from 4 to 5

Bumps [actions/checkout](https://github.com/actions/checkout) from 4 to 5.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v4...v5)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '5'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

.github/workflows/linter.yaml

@@ -12,7 +12,7 @@ jobs:
     if: github.repository == 'a2aproject/a2a-python'
     steps:
       - name: Checkout Code
-        uses: actions/checkout@v4
+        uses: actions/checkout@v5
       - name: Set up Python
         uses: actions/setup-python@v5
         with:

.github/workflows/python-publish.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v5
 
       - name: Install uv
         uses: astral-sh/setup-uv@v5

.github/workflows/unit-tests.yml

@@ -39,7 +39,7 @@ jobs:
         python-version: ['3.10', '3.13']
     steps:
       - name: Checkout code
-        uses: actions/checkout@v4
+        uses: actions/checkout@v5
       - name: Set up test environment variables
         run: |
           echo "POSTGRES_TEST_DSN=postgresql+asyncpg://a2a:a2a_password@localhost:5432/a2a_test" >> $GITHUB_ENV

.github/workflows/update-a2a-types.yml

@@ -12,7 +12,7 @@ jobs:
       pull-requests: write
     steps:
       - name: Checkout code
-        uses: actions/checkout@v4
+        uses: actions/checkout@v5
       - name: Set up Python
         uses: actions/setup-python@v5
         with: