chore: Add GitHub Action check to run mypy

Merge https://github.com/google/adk-python/pull/4276

This adds 2 mypy GitHub Actions checks:

* `mypy.yml` runs mypy on every PR. This will list all errors, so it shouldn't block until errors are resolved.
* `mypy-new-errors.yml` compares the PR branch to `main` to ensure no new mypy errors are introduced. This check can be made blocking now and can be removed once all errors are resolved in favor of `mypy.yml`.

### Checklist

- [x] I have read the [CONTRIBUTING.md](https://github.com/google/adk-python/blob/main/CONTRIBUTING.md) document.
- [x] I have performed a self-review of my own code.
- [x] I have commented my code, particularly in hard-to-understand areas.
- [x] I have added tests that prove my fix is effective or that my feature works.
- [x] New and existing unit tests pass locally with my changes.
- [x] I have manually tested my changes end-to-end.
- [x] Any dependent changes have been merged and published in downstream modules.

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/4276 from sararob:mypy-gh-actions 63130773c4862e90e352d4cb79221d78a6b9f5fd
PiperOrigin-RevId: 862226296

.github/workflows/mypy-new-errors.yml

@@ -0,0 +1,74 @@
+name: Mypy New Error Check
+
+on:
+  push:
+    branches: [ main ]
+  pull_request:
+    branches: [ main ]
+
+
+jobs:
+  mypy-diff:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python-version: ['3.10', '3.11', '3.12', '3.13',]
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: ${{ matrix.python-version }}
+
+      - name: Install uv
+        uses: astral-sh/setup-uv@v5
+
+      - name: Generate Baseline (Main)
+        run: |
+          # Switch to main branch to generate baseline
+          git checkout origin/main
+
+          git checkout ${{ github.sha }} -- pyproject.toml
+          
+          # Install dependencies for main
+          uv venv .venv
+          source .venv/bin/activate
+          uv sync --all-extras
+          
+          # Run mypy, filter for errors only, remove line numbers (file:123: -> file::), and sort
+          # We ignore exit code (|| true) because we expect errors on main
+          uv run mypy . | grep "error:" | sed 's/:\([0-9]\+\):/::/g' | sort > main_errors.txt || true
+          
+          echo "Found $(wc -l < main_errors.txt) errors on main."
+
+      - name: Check PR Branch
+        run: |
+          # Switch back to the PR commit
+          git checkout ${{ github.sha }}
+          
+          # Re-sync dependencies in case the PR changed them
+          source .venv/bin/activate
+          uv sync --all-extras
+          
+          # Run mypy on PR code, apply same processing
+          uv run mypy . | grep "error:" | sed 's/:\([0-9]\+\):/::/g' | sort > pr_errors.txt || true
+          
+          echo "Found $(wc -l < pr_errors.txt) errors on PR branch."
+
+      - name: Compare and Fail on New Errors
+        run: |
+          # 'comm -13' suppresses unique lines in file1 (main) and common lines, 
+          # leaving only lines unique to file2 (PR) -> The new errors.
+          comm -13 main_errors.txt pr_errors.txt > new_errors.txt
+          
+          if [ -s new_errors.txt ]; then
+            echo "::error::The following NEW mypy errors were introduced:"
+            cat new_errors.txt
+            exit 1
+          else
+            echo "Great job! No new mypy errors introduced."
+          fi

.github/workflows/mypy.yml

@@ -0,0 +1,32 @@
+name: Mypy Type Check
+
+on:
+  push:
+    branches: [ main ]
+  pull_request:
+    branches: [ main ]
+
+jobs:
+  mypy:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python-version: ['3.10', '3.11', '3.12', '3.13',]
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install uv
+        uses: astral-sh/setup-uv@v1
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: ${{ matrix.python-version }}
+
+      - name: Install dependencies
+        run: uv sync --all-extras
+
+      - name: Run mypy
+
+        run: uv run mypy . --strict

pyproject.toml

@@ -216,9 +216,9 @@ asyncio_mode = "auto"
 
 [tool.mypy]
 python_version = "3.10"
-exclude = "tests/"
+exclude = ["tests/", "contributing/samples/"]
 plugins = ["pydantic.mypy"]
 # Start with non-strict mode, and swtich to strict mode later.
-# strict = true
+strict = true
 disable_error_code = ["import-not-found", "import-untyped", "unused-ignore"]
 follow_imports = "skip"