Reject v1-style flag aliases

[ibobgunardi]

Fixes #1103.

This makes the v2 branch fail fast when a built-in flag is still written with the old v1-style Name value, for example Name: "config, cfg" or Name: "config cfg". The error points developers to Aliases instead of silently registering only the first name.

I also updated two existing tests that were still using the old form so they keep testing aliases through the supported API.

Tests:

• go test . -run 'TestAppRunRejectsV1StyleFlagNames|TestCommandRunRejectsV1StyleFlagNames|TestCheckRequiredFlags|ExampleApp_Run_bashComplete_withLongFlag' -count=1
• go test . -count=1
• go test ./... -count=1
• go vet ./...

[ibobgunardi]

Checked the red jobs after the update.

• Package tests pass locally with go test ./... -count=1, and go vet ./... passes.
• The lint job fails before reporting a repo file: golangci-lint v1.64.8 is typechecking a Go 1.26 stdlib file and exits with file requires newer Go version go1.26 (application built with go1.24).
• The Ubuntu job gets through the tests/builds and fails at make check-binary-size. I also ran go run internal/build/build.go check-binary-size on a clean origin/v2-maint checkout with the current local Go toolchain, and that baseline fails the same threshold. I removed the reflection-based version of this patch; the current branch only adds about 1.5KB to internal/example-cli in my local byte comparison and leaves internal/example-hello-world unchanged.

I am leaving the binary-size threshold alone here so this PR stays focused on #1103.

[dearchap]

@ibobgunardi v2 is in maint with only security patches.

[ibobgunardi]

Thanks, that makes sense. This is not security-related, so I'm closing it rather than keeping a v2-maint PR open.