ci: disable fullyparallel for unsharded weekly Postgres jobs (#36390)

Restore the `fullyparallel: false` override for the unsharded
`Postgres with binary parameters` and `Postgres FIPS` jobs in the
weekly workflow. The override was originally added to the binary
parameters job in #35995 to prevent resource exhaustion on a single
runner, but was dropped when both jobs moved into
server-ci-weekly.yml in #36036, leaving them on the template default
of `true`.

Without it, the hosted runner is overwhelmed (too many server
instances, WebSocket hubs, and DB connections) and the runner agent
itself loses communication with GitHub mid-run, surfacing as
"hosted runner lost communication with the server" at ~55-60 min
into the Run Tests step. Both runs on April 27 and May 4 failed
this way; the sharded FIPS variant retained for FIPS-touching PRs
in server-ci.yml is unaffected because each shard handles only a
fraction of the packages.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/server-ci-weekly.yml

@@ -47,6 +47,10 @@ jobs:
       logsartifact: postgres-binary-server-test-logs
       go-version: ${{ needs.go.outputs.version }}
       fips-enabled: false
+      # Unsharded run on a single 8-core runner: fullyparallel=true causes
+      # resource exhaustion (too many server instances, WebSocket hubs, and
+      # DB connections) and crashes the hosted runner. See #35995.
+      fullyparallel: false
 
   test-postgres-normal-fips:
     name: Postgres FIPS
@@ -60,6 +64,8 @@ jobs:
       logsartifact: postgres-server-fips-test-logs
       go-version: ${{ needs.go.outputs.version }}
       fips-enabled: true
+      # Unsharded run on a single 8-core runner: see note on test-postgres-binary.
+      fullyparallel: false
 
   test-mmctl-fips:
     name: Run mmctl tests (FIPS)