test(bigtable): compose bigtable-session-it with cfe/directpath profiles and verify VirtualRpc traffic - #14963

Summary

• Make bigtable-session-it a composable Maven profile that sets bigtable.internal.client-config-override-use-default=true and bigtable.internal.client-config-override=session_configuration { session_diversion_configuration { global_diversion { session_load: 1.0 } } } so it can be combined with bigtable-cfe-it (-P bigtable-cfe-it,bigtable-session-it) or bigtable-directpath-it (-P bigtable-directpath-it,bigtable-session-it).
• Scope bigtable-cfe-it failsafe <includes> to com.google.cloud.bigtable.data.v2.it.*IT (matching bigtable-directpath-it), which includes SessionIT when bigtable-session-it is active (and skips SessionIT via Assume when bigtable-session-it is not active).
• Update .kokoro/presubmit/bigtable-integration-session.cfg to compose -P bigtable-cfe-it,bigtable-session-it,enable-verbose-grpc-logs.
• Update SessionIT:
  • Chain PeerInfoCapturingInterceptor onto any existing channelConfigurator from CloudEnv (REQUIRE_CFE / REQUIRE_DIRECT_PATH) instead of overwriting it.
  • Assert that readRow and mutateRow actually send SessionRequest.virtual_rpc messages over OpenTable and do not fall back to classic ReadRows/MutateRow RPCs (previously SessionIT only checked bigtable-peer-info headers from background OpenTable stream establishment, allowing silent fallback to classic RPCs to go undetected).
  • Assert the expected PeerInfo.TransportType (TRANSPORT_TYPE_SESSION_DIRECT_ACCESS under REQUIRE_DIRECT_PATH, TRANSPORT_TYPE_SESSION_CLOUD_PATH under REQUIRE_CFE).

Related PRs

• Depends on fix(bigtable): avoid triggering per-RPC credential session fallback on default call context #14961 (expected to fail SessionIT on main until fix(bigtable): avoid triggering per-RPC credential session fallback on default call context #14961 is merged).