feat(server): add proactive channel probes and load-aware stable routing / 服务端新增主动探活与负载感知稳定路由 (#330)

* 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

* feat(server): add proactive channel probes and load-aware stable routing

* test(server): stabilize session load routing selection assertion

* fix(proxy): harden codex forwarding stability

* fix(router): rotate stable-first channels by healthy order

* fix(server): address probe review feedback

* fix(server): address probe review follow-ups

* fix(ci): address CodeQL and core test failures

* feat(server): refine stable-first routing recovery and observation

* Finish PR 330 settings and review fixes

* Fix chat site endpoint CI mock

* Address latest PR 330 review comments

---------

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

.env.example

@@ -8,6 +8,10 @@ SERVERCHAN_KEY=
 TELEGRAM_ENABLED=false
 TELEGRAM_BOT_TOKEN=
 TELEGRAM_CHAT_ID=
+MODEL_AVAILABILITY_PROBE_CONCURRENCY=1
+MODEL_AVAILABILITY_PROBE_ENABLED=false
+MODEL_AVAILABILITY_PROBE_INTERVAL_MS=1800000
+MODEL_AVAILABILITY_PROBE_TIMEOUT_MS=15000
 PORT=4000
 DATA_DIR=./data
 NOTIFY_COOLDOWN_SEC=300