fix: migrate HTTP/2 transport to the standard library

[ryanfowler]

Summary

• Replace deprecated golang.org/x/net/http2.Transport usage with Go 1.27 net/http.Transport HTTP/2 protocol configuration.
• Preserve direct HTTPS HTTP/2, h2c, CONNECT proxying, proxy selection, custom dialing, TLS/ECH, and scheme validation behavior.
• Add end-to-end coverage for direct and proxied HTTPS HTTP/2 and h2c.
• Bump golang.org/x/net to 0.59.0 as required by PR chore(deps): bump golang.org/x/net from 0.58.0 to 0.59.0 #1188.

Why

Staticcheck reports SA1019 for the deprecated x/net/http2.Transport in PR #1188. The standard library transport is the supported replacement in Go 1.27.

Validation

• go test -v -parallel 8 ./...
• go test -race -count=1 -parallel 8 ./internal/client ./internal/fetch ./internal/grpc ./internal/pager ./internal/resolver ./internal/ws
• staticcheck ./...
• go mod verify
• native and CGO-disabled release builds for Linux, macOS, and Windows targets

Independent review found no correctness defects.