feat: add celeristest.WithHandlers for middleware chain testing

[FumingPower3925]

Summary

• Add celeristest.WithHandlers(...HandlerFunc) option to set a handler chain on test contexts
• Add ctxkit.SetHandlers internal hook to bridge the type boundary
• Register SetHandlers in context.go init block

This enables the goceleris/middlewares v0.1.0 testutil.RunChain helper, which tests nested middleware (e.g. recovery wrapping a panicking handler, logger timing downstream).

Closes #139

Changes

File	Change
internal/ctxkit/ctxkit.go	Add SetHandlers func(c any, handlers []any) hook
context.go	Register hook: converts []any → []HandlerFunc, sets on context
celeristest/celeristest.go	Add handlers []any to config, WithHandlers option, apply in NewContext

Test plan

• go test ./celeristest/... passes
• go build ./... passes
• Verified via goceleris/middlewares testutil.RunChain — all 9 packages pass with -race