fix(tests): unstick CI — sweep stale tests from recent merges (#12670)

One source fix (web_server category merge) + five test updates that
didn't travel with their feature PRs. All 13 failures on the 04-19
CI run on main are now accounted for (5 already self-healed on main;
8 fixed here).

Changes
- web_server.py: add code_execution → agent to _CATEGORY_MERGE (new
  singleton section from #11971 broke no-single-field-category invariant).
- test_browser_camofox_state: bump hardcoded _config_version 18 → 19
  (also from #11971).
- test_registry: add browser_cdp_tool (#12369) and discord_tool (#4753)
  to the expected built-in tool set.
- test_run_agent::test_tool_call_accumulation: rewrite fragment chunks
  — #0f778f77 switched streaming name-accumulation from += to = to
  fix MiniMax/NIM duplication; the test still encoded the old
  fragment-per-chunk premise.
- test_concurrent_interrupt::_Stub: no-op
  _apply_pending_steer_to_tool_results — #12116 added this call after
  concurrent tool batches; the hand-rolled stub was missing it.
- test_codex_cli_model_picker: drop the two obsolete tests that
  asserted auto-import from ~/.codex/auth.json into the Hermes auth
  store. #12360 explicitly removed that behavior (refresh-token reuse
  races with Codex CLI / VS Code); adoption is now explicit via
  `hermes auth openai-codex`. Remaining 3 tests in the file (normal
  path, Claude Code fallback, negative case) still cover the picker.

Validation
- scripts/run_tests.sh across all 6 affected files + surrounding tests
  (54 tests total) all green locally.

hermes_cli/web_server.py

@@ -234,6 +234,7 @@ _CATEGORY_MERGE: Dict[str, str] = {
     "human_delay": "display",
     "smart_model_routing": "agent",
     "dashboard": "display",
+    "code_execution": "agent",
 }
 
 # Display order for tabs — unlisted categories sort alphabetically after these.