ci: verify go.mod files all have the same declared go version (#1790)

## Why this should be merged

Closes: #1789

## How this works

This adds a CI workflow to ensure the go version in `go.mod` is the same
for all tracked files.

## How this was tested

CI:
https://github.com/ava-labs/firewood/actions/runs/23058274067/job/66977621517

## Breaking Changes

None

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: demosdemon

.github/workflows/verify-go-versions.yaml

@@ -0,0 +1,46 @@
+name: Verify Go Versions
+
+on:
+  pull_request:
+  push:
+    branches: [main]
+
+jobs:
+  verify-go-versions:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v6
+
+      - name: Install jq
+        run: |
+          if command -v jq >/dev/null 2>&1; then
+            # expected on most GHA runners
+            echo "jq is already installed"
+          else
+           sudo apt-get update && sudo apt-get install -y jq
+          fi
+
+      - name: Extract Go versions
+        id: go_versions
+        run: |
+          GO_VERSION_FFI=$(cd ffi && go mod edit -json | jq -r .Go)
+          GO_VERSION_TEST_ETH=$(cd ffi/tests/eth && go mod edit -json | jq -r .Go)
+          GO_VERSION_TEST_FIREWOOD=$(cd ffi/tests/firewood && go mod edit -json | jq -r .Go)
+          echo "ffi=$GO_VERSION_FFI"
+          echo "ffi/tests/eth=$GO_VERSION_TEST_ETH"
+          echo "ffi/tests/firewood=$GO_VERSION_TEST_FIREWOOD"
+          echo "GO_VERSION_FFI=$GO_VERSION_FFI" >> $GITHUB_OUTPUT
+          echo "GO_VERSION_TEST_ETH=$GO_VERSION_TEST_ETH" >> $GITHUB_OUTPUT
+          echo "GO_VERSION_TEST_FIREWOOD=$GO_VERSION_TEST_FIREWOOD" >> $GITHUB_OUTPUT
+
+      - name: Check Go versions match
+        run: |
+          if [ "${{ steps.go_versions.outputs.GO_VERSION_FFI }}" != "${{ steps.go_versions.outputs.GO_VERSION_TEST_ETH }}" ] || \
+             [ "${{ steps.go_versions.outputs.GO_VERSION_FFI }}" != "${{ steps.go_versions.outputs.GO_VERSION_TEST_FIREWOOD }}" ]; then
+            echo "Go versions do not match:"
+            echo "ffi/go.mod: ${{ steps.go_versions.outputs.GO_VERSION_FFI }}"
+            echo "ffi/tests/eth/go.mod: ${{ steps.go_versions.outputs.GO_VERSION_TEST_ETH }}"
+            echo "ffi/tests/firewood/go.mod: ${{ steps.go_versions.outputs.GO_VERSION_TEST_FIREWOOD }}"
+            exit 1
+          fi
+          echo "All Go versions match: ${{ steps.go_versions.outputs.GO_VERSION_FFI }}"

ffi/go.mod

@@ -1,12 +1,10 @@
 module github.com/ava-labs/firewood/ffi
 
-go 1.25
-
-// Changes to the toolchain version should be replicated in:
+// Changes to the go version should be replicated in:
 //   - ffi/go.mod (here)
 //   - ffi/tests/eth/go.mod
 //   - ffi/tests/firewood/go.mod
-toolchain go1.25.8
+go 1.25.8
 
 require (
 	github.com/prometheus/client_golang v1.22.0

ffi/tests/eth/go.mod

@@ -1,8 +1,6 @@
 module github.com/ava-labs/firewood/ffi/tests/eth
 
-go 1.25
-
-toolchain go1.25.8
+go 1.25.8
 
 require (
 	github.com/ava-labs/firewood-go-ethhash/ffi v0.0.0 // this is replaced to use the parent folder

ffi/tests/firewood/go.mod

@@ -1,8 +1,6 @@
 module github.com/ava-labs/firewood/ffi/tests/firewood
 
-go 1.25
-
-toolchain go1.25.8
+go 1.25.8
 
 require (
 	github.com/ava-labs/firewood-go/ffi v0.0.0 // this is replaced to use the parent folder