test(ci): bound concurrent extension host fixtures

Windows full-suite runs on PRs6776 and6780 reported normal extension hosts missing the production five-second handshake deadline. Serialize the host fixture family across nextest processes without changing production deadlines, hang-detection tests, or retries. Nextest profile ci resolves all39 tests (35 extension-host module and4 engine callers) to max-threads1; governed focused run39/39 passed in28.901s on macOS. Exact config SHA256456e681709fe5838e404fbf537f6a934d700d0577ec15bf6a18eeae17aecde53. Full npm740/0 and check:web passed. Windows resolution still requires new hosted runs.

Signed-off-by: Hunter B <[email protected]>

.config/nextest.toml

@@ -44,6 +44,11 @@ exec-persistent-service = { max-threads = 1 }
 # (a durable heartbeat and a `ps` sample on every scheduler tick) is now
 # bounded to once a second per worker.
 fleet-manager-lifecycle = { max-threads = 1 }
+# Extension fixtures launch real Node hosts with a five-second production
+# handshake deadline. Windows full-suite runs on #6776/#6780 reported ordinary
+# hosts missing that deadline. Bound contention across nextest processes;
+# keep production deadlines and the explicit hang-detection tests unchanged.
+extension-host = { max-threads = 1 }
 
 # First matching test-group override wins. Keep these more-specific
 # integration filters before binary(integration) so they are not stolen
@@ -70,6 +75,10 @@ test-group = 'telemetry-contract'
 filter = 'package(codewhale-tui) & kind(lib) & test(/^fleet::manager::tests::/)'
 test-group = 'fleet-manager-lifecycle'
 
+[[profile.default.overrides]]
+filter = 'package(codewhale-tui) & kind(lib) & (test(/^extension_host::/) | test(/^core::engine::.*extension/))'
+test-group = 'extension-host'
+
 [[profile.default.overrides]]
 filter = 'binary(integration) & test(/^exec_persistent_service::/)'
 test-group = 'exec-persistent-service'