fix: resolve P0/P1 code quality issues

P0 fixes:
- ModelError.Is(): use exact matching instead of substring contains()
- shouldClearStickySession: add context param for cancellation/tracing

P1 fixes:
- TODO stubs: return 501 Not Implemented errors
- validateInstanceSignature: deduplicate to shared validateCodeSignature()
- Error messages: standardize to English only
- http.go: remove pseudo if-else with duplicate branches

backend/ent/driver_access.go

@@ -0,0 +1,8 @@
+package ent
+
+import "entgo.io/ent/dialect"
+
+// Driver 暴露底层 driver，供需要 raw SQL 的集成层使用。
+func (c *Client) Driver() dialect.Driver {
+	return c.driver
+}