test(example-event-handlers): pin in-container hardfork to cancun

The hardhat node in examples/ethereum-basic-event-handlers/hardhat
runs with `"hardhat": "^2.22.1"` and no lockfile, so each CI run
picks the latest matching release (currently 2.28.6). Hardhat 2.26+
defaults to the Prague hardfork, which enforces EIP-7825's per-
transaction gas cap of 2^24 (16,777,216). The test contract deploy
needs ~30M gas and gets rejected with `ProviderError: Transaction
gas limit is 30000000 and exceeds transaction gas cap of 16777216`.

Pinning the in-container network's hardfork to `cancun` restores
pre-EIP-7825 behavior without locking the hardhat version.

Author: lutter

examples/ethereum-basic-event-handlers/hardhat/hardhat.config.js

@@ -1,3 +1,10 @@
 module.exports = {
   solidity: '0.8.9',
+  networks: {
+    hardhat: {
+      // Pin to Cancun: Prague (default in hardhat >=2.26) enforces EIP-7825's
+      // per-transaction gas cap of 2^24, which rejects the test deployments.
+      hardfork: 'cancun',
+    },
+  },
 };