fix(ci): split release chat verification into test shards (#13198)

Split release chat verification into three validated test-line shards and balance other server suites across five runners using the measured native Runner integration cost. Retire each chat case's fixtures after assertions, preserve complete test coverage, and exercise the real shard CLI in PR tests.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

.github/workflows/release-verify.yml

@@ -58,30 +58,41 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          # Five-way server split, matching pr-trusted.yml. Three shards sat
-          # at 17-19 minutes against this job's 20-minute cap after the
-          # server suite grew on 2026-09-10, and every canary that evening
-          # died on the timeout instead of reporting a verdict.
-          - group: general-server
+          # Split the long chat file by collected test locations, and balance
+          # the remaining server files across five runners. Normal PR/local
+          # invocations retain their complete general-server group.
+          - group: general-server-without-chat
             group_label: server (1/5)
             shard_index: 0
             shard_count: 5
-          - group: general-server
+          - group: general-server-without-chat
             group_label: server (2/5)
             shard_index: 1
             shard_count: 5
-          - group: general-server
+          - group: general-server-without-chat
             group_label: server (3/5)
             shard_index: 2
             shard_count: 5
-          - group: general-server
+          - group: general-server-without-chat
             group_label: server (4/5)
             shard_index: 3
             shard_count: 5
-          - group: general-server
+          - group: general-server-without-chat
             group_label: server (5/5)
             shard_index: 4
             shard_count: 5
+          - group: general-chat
+            group_label: chat (1/3)
+            shard_index: 0
+            shard_count: 3
+          - group: general-chat
+            group_label: chat (2/3)
+            shard_index: 1
+            shard_count: 3
+          - group: general-chat
+            group_label: chat (3/3)
+            shard_index: 2
+            shard_count: 3
           # Keep parity with pr.yml: workspaces-a is split with Vitest's
           # native --shard because the ui project dominates the lane.
           - group: general-workspaces-a