ci: pin CI to Python 3.13, remove version matrix (anthropics#537)

## Summary
- Remove the `python-version` matrix from `test`, `test-e2e`, and
`test-examples` CI jobs
- Pin all jobs to Python 3.13 directly instead of testing across 3.10,
3.11, 3.12, and 3.13
- Reduces CI matrix size and speeds up workflow runs

## Test plan
- [ ] Verify CI passes on all three OS targets (ubuntu, macos, windows)
with Python 3.13
- [ ] Confirm unit tests, e2e tests, and example tests all run
successfully

## Changelog
<!-- CHANGELOG:START -->
<!-- CHANGELOG:END -->

🤖 Generated with [Claude Code](https://claude.com/claude-code) (0%
1-shotted by claude)

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: ashwin-ant

.github/workflows/test.yml

@@ -12,15 +12,14 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        python-version: ["3.10", "3.11", "3.12", "3.13"]
 
     steps:
       - uses: actions/checkout@v4
 
-      - name: Set up Python ${{ matrix.python-version }}
+      - name: Set up Python
         uses: actions/setup-python@v5
         with:
-          python-version: ${{ matrix.python-version }}
+          python-version: "3.13"
 
       - name: Install dependencies
         run: |
@@ -45,15 +44,14 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        python-version: ["3.10", "3.11", "3.12", "3.13"]
 
     steps:
       - uses: actions/checkout@v4
 
-      - name: Set up Python ${{ matrix.python-version }}
+      - name: Set up Python
         uses: actions/setup-python@v5
         with:
-          python-version: ${{ matrix.python-version }}
+          python-version: "3.13"
 
       - name: Install Claude Code (Linux/macOS)
         if: runner.os == 'Linux' || runner.os == 'macOS'
@@ -108,17 +106,14 @@ jobs:
     if: github.event_name == 'push' || github.event.pull_request.head.repo.full_name == github.repository
     runs-on: ubuntu-latest
     needs: test-e2e # Run after e2e tests
-    strategy:
-      matrix:
-        python-version: ["3.13"]
 
     steps:
       - uses: actions/checkout@v4
 
-      - name: Set up Python ${{ matrix.python-version }}
+      - name: Set up Python
         uses: actions/setup-python@v5
         with:
-          python-version: ${{ matrix.python-version }}
+          python-version: "3.13"
 
       - name: Install Claude Code (Linux)
         if: runner.os == 'Linux'