ci: add nightly external contract tests

.github/workflows/external-contracts.yml

@@ -0,0 +1,30 @@
+name: External contracts
+
+on:
+  schedule:
+    - cron: "0 3 * * *"
+  workflow_dispatch:
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+env:
+  CARGO_PROFILE_TEST_DEBUG: line-tables-only
+  CARGO_INCREMENTAL: 0
+
+jobs:
+  external-contracts:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6
+
+      - uses: dtolnay/rust-toolchain@29eef336d9b2848a0b548edc03f92a220660cdb8 # stable
+
+      - uses: Swatinem/rust-cache@e18b497796c12c097a38f9edb9d0641fb99eee32 # v2
+
+      # These tests intentionally call real third-party APIs. Unit/integration
+      # tests mock providers, but a nightly contract check catches external API
+      # drift that would otherwise only appear at runtime.
+      - name: Run external contract tests
+        run: cargo test --locked --test external_contract -- --ignored --test-threads=1