test: add comprehensive unit tests for provider, middleware, and sync service

- Add TESTING.md documentation explaining unit test conventions and integration testing setup
- Add miniredis and sqlite dependencies to go.mod for in-memory testing
- Add provider_handler_test.go ensuring Vertex providers default google_location to "global"
- Add request_id_test.go verifying request ID generation and header preservation
- Add sync_test.go validating Redis snapshot writes and routing key generation
- Update README.md with testing section referencing new documentation

go.mod

@@ -3,6 +3,7 @@ module github.com/ez-api/ez-api
 go 1.24.5
 
 require (
+	github.com/alicebob/miniredis/v2 v2.35.0
 	github.com/ez-api/foundation v0.1.0
 	github.com/gin-gonic/gin v1.11.0
 	github.com/redis/go-redis/v9 v9.17.2
@@ -12,6 +13,7 @@ require (
 	github.com/swaggo/swag v1.16.6
 	golang.org/x/crypto v0.44.0
 	gorm.io/driver/postgres v1.6.0
+	gorm.io/driver/sqlite v1.6.0
 	gorm.io/gorm v1.31.1
 )
 
@@ -52,6 +54,7 @@ require (
 	github.com/leodido/go-urn v1.4.0 // indirect
 	github.com/mattn/go-colorable v0.1.13 // indirect
 	github.com/mattn/go-isatty v0.0.20 // indirect
+	github.com/mattn/go-sqlite3 v1.14.22 // indirect
 	github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd // indirect
 	github.com/modern-go/reflect2 v1.0.2 // indirect
 	github.com/pelletier/go-toml/v2 v2.2.4 // indirect
@@ -66,6 +69,7 @@ require (
 	github.com/subosito/gotenv v1.6.0 // indirect
 	github.com/twitchyliquid64/golang-asm v0.15.1 // indirect
 	github.com/ugorji/go/codec v1.3.0 // indirect
+	github.com/yuin/gopher-lua v1.1.1 // indirect
 	go.uber.org/mock v0.5.0 // indirect
 	go.yaml.in/yaml/v3 v3.0.4 // indirect
 	golang.org/x/arch v0.20.0 // indirect