[feature][bug] Add tcp transport event dispatcher unsched flag & fix RDMA event dispatcher unsched flag

[MalikHou]

Event Dispatcher Unsched Flag

What problem does this PR solve?

Problem Summary:

• Event-dispatch scheduling behavior was not unified across transports.
• RDMA used a transport-specific flag (rdma_edisp_unsched), while TCP had no unified switch.
• In RDMA path, bthread_start_urgent and bthread_start_background logic was reversed, causing behavior opposite to expected unsched semantics.
• The scheduling decision logic was duplicated in transport code, making behavior harder to reason about and test.

What is changed and the side effects?

Changed:

• Added a unified flag in dispatcher layer: event_dispatcher_edisp_unsched.
• Added EventDispatcherUnsched() helper in event_dispatcher.{h,cpp} as single source of truth.
• Updated TcpTransport::ProcessEvent and RdmaTransport::ProcessEvent to use EventDispatcherUnsched():
  • false -> bthread_start_urgent
  • true -> bthread_start_background
• Fixed RDMA bug where bthread_start_urgent and bthread_start_background branches were previously swapped.
• Deprecated rdma_edisp_unsched and kept compatibility through dispatcher-layer unified check.
• Added/updated tests in test/brpc_event_dispatcher_unittest.cpp:
  • event_dispatcher_unsched_by_unified_flag
  • event_dispatcher_unsched_by_legacy_rdma_flag (when BRPC_WITH_RDMA)
  • tcp_unsched_true_returns_before_onedge_finish
  • tcp_unsched_false_blocks_caller_when_single_worker

Effects:

• event_dispatcher_edisp_unsched=true may reduce immediate foreground switching of dispatcher path.
• rdma_edisp_unsched is deprecated.
• RDMA runtime behavior now matches intended unsched semantics after fixing the swapped urgent/background branches. Users depending on previous incorrect behavior should migrate and re-check tuning.

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Check List:

• Please make sure your changes are compilable.
• When providing us with a new feature, it is best to add related tests.
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