feat: add PR validation workflow

The Python SDK had no pull_request-triggered checks, so regen PRs
(speakeasy-sdk-regen-*) carried an empty status-check rollup and the
auto-merge flow would merge them with zero validation. The TypeScript
SDK validates every regen PR via pr-validation.yaml; Python had no
equivalent.

Add a single-file PR Validation workflow that builds the package and
runs the SDK's existing quality gates (mypy, pyright, pylint) via uv.
No tests exist in this repo yet, so no API-key secret is needed, which
also lets the workflow run on fork PRs. paths-ignore on
.speakeasy/in.openapi.yaml mirrors TS so pure spec-bump PRs don't
double-trigger.

This registers a `validate` check that the auto-merge wait_for_checks
step will gate on once both land.

.github/workflows/pr-validation.yaml

@@ -0,0 +1,37 @@
+name: PR Validation
+
+on:
+  pull_request:
+    paths-ignore:
+      - .speakeasy/in.openapi.yaml
+
+concurrency:
+  group: pr-validation-${{ github.event.pull_request.number }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+
+jobs:
+  validate:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v5
+
+      - name: Install uv
+        uses: astral-sh/setup-uv@v5
+        with:
+          enable-cache: true
+
+      - name: Build SDK
+        run: uv build
+
+      - name: Type check (mypy)
+        run: uv run --group dev mypy src
+
+      - name: Type check (pyright)
+        run: uv run --group dev pyright src
+
+      - name: Lint (pylint)
+        run: uv run --group dev pylint src --rcfile pylintrc