fix: error on legacy v1-style flag alias syntax

[happysnaker]

Fixes #1103

This changes legacy v1-style flag alias definitions like Name: "config, cfg" from a silent partial-accept path into an explicit error.

What this patch does:

• validates Name / Aliases before registering each flag
• rejects commas or spaces inside the primary Name field
• rejects malformed aliases that still contain commas or spaces
• adds a regression test that asserts App.Run() errors with a migration hint
• updates the long-flag completion example to use v2-style Aliases explicitly

Why this shape:

• today the old syntax looks half-working: the primary flag still exists, but the alias silently disappears
• failing fast is easier to debug and matches the issue request for an error/warning rather than hidden misconfiguration
• validating in each flag Apply() path keeps the behavior consistent for direct flag registration and wrappers like altsrc that delegate to the same Apply() methods

I intentionally kept the error message migration-oriented rather than trying to preserve the old behavior.