Run route middleware for OPTIONS fallbacks

[puneetdixit200]

Overview:

• Preserve route middleware on the router's implicit OPTIONS fallback for an existing path.
• Use Access-Control-Request-Method to choose the matched route middleware when handling browser preflights.
• Add a regression test for grouped CORS middleware without an explicit OPTIONS route.

What problem does it solve?:
Closes #2950. In v5, grouped CORS middleware could be skipped for preflight requests when the group had a matching GET route but no explicit OPTIONS route. The router returned its implicit OPTIONS handler directly, so the CORS middleware never added Access-Control-Allow-Origin. This keeps the fallback handler but wraps it with the matched route middleware chain, allowing CORS to handle the preflight before the fallback writes the 204 response.

Special notes for a reviewer:
Verified with:

• GOMODCACHE=/tmp/echo-gomodcache GOCACHE=/tmp/echo-gocache go test ./middleware -run TestCORSWithConfig_GroupPreflightWithoutOptionsRoute -count=1
• GOMODCACHE=/tmp/echo-gomodcache GOCACHE=/tmp/echo-gocache go test ./...
• GOMODCACHE=/tmp/echo-gomodcache GOCACHE=/tmp/echo-gocache go vet ./...
• GOMODCACHE=/tmp/echo-gomodcache GOCACHE=/tmp/echo-gocache go test -race ./...
• PATH=/tmp/echo-bin:$PATH GOMODCACHE=/tmp/echo-gomodcache GOCACHE=/tmp/echo-gocache staticcheck ./...
• PATH=/tmp/echo-bin:$PATH GOMODCACHE=/tmp/echo-gomodcache GOCACHE=/tmp/echo-gocache golint -set_exit_status ./...
• git diff --check