feat(proxy): improve channel failover stability and sticky session coordination / 优化代理通道故障切换与粘滞会话协调 (#268)

* wip: preserve local changes before upstream merge

* Improve proxy routing stability and add stable-first strategy

* Sync explicit-group routing strategy to eligible source routes

* Fix proxy retry and runtime health review issues

* fix: make proxy channel retry attempts configurable

* fix: harden proxy retry recovery and bookkeeping

* fix: address pr 257 review comments

* feat(proxy): add sticky session channel coordination

---------

Co-authored-by: Cita <juricek.chen@gmail.com>

src/server/routes/proxy/endpointFlow.test.ts

@@ -298,7 +298,6 @@ describe('executeEndpointFlow', () => {
 
     expect(fetchMock.mock.calls[0]?.[0]).toBe('https://proxy.internal/base/v1/responses');
   });
-
   it('normalizes proxyUrl with versioned base paths instead of duplicating path segments', async () => {
     fetchMock.mockResolvedValueOnce(toUndiciResponse(new Response(JSON.stringify({ ok: true }), {
       status: 200,