mcp: re-enable race test after fixing data races (#851)

1. (non-test) Don't queue notifications if there are no connected
sessions.
2. Change `reconnectInitialDelay` to be atomic, as it is modified in
tests.
3. Protect map access with mutex in `TestServerTransportCleanup`.

.github/workflows/test.yml

@@ -18,7 +18,7 @@ jobs:
       - name: Set up Go
         uses: actions/setup-go@4b73464bb391d4059bd26b0524d20df3927bd417 # v6.3.0
         with:
-          go-version: "^1.25"
+          go-version: "^1.26"
       - name: Check formatting
         run: |
           unformatted=$(gofmt -l .)
@@ -52,8 +52,6 @@ jobs:
         run: go test -v ./...
 
   race-test:
-    # Temporarily disable until fixes are prepared.
-    if: false
     runs-on: ubuntu-latest
     steps:
       - name: Check out code
@@ -61,6 +59,6 @@ jobs:
       - name: Set up Go
         uses: actions/setup-go@4b73464bb391d4059bd26b0524d20df3927bd417 # v6.3.0
         with:
-          go-version: "1.25"
+          go-version: "1.26"
       - name: Test with -race
         run: go test -v -race ./...