feat: validate Flux bootstrap flags before creating the cluster

[JesusZuluaga-M]

Description

Closes #3664

I think this may resolve this issue.

Currently eksctl create cluster only discovers invalid gitops.flux.flags when
it calls flux bootstrap at the very end of the process — after the cluster has
already taken ~25 minutes to provision. This PR validates those flags upfront,
before cluster creation starts.

Flux has no dry-run mode for bootstrap, so the validation uses the workaround
suggested in the issue: it builds the same args Bootstrap() would use and appends
a sentinel flag that Flux can never recognise (--eksctl-internal-validate-only).
Since Flux's flag parser (pflag) stops at the first unrecognised flag:

• if the resulting error mentions the sentinel flag → all real flags were valid
• if it mentions anything else → that's the real flag that's misconfigured

Checklist

• Added tests that cover your change (if possible)
• Added/modified documentation as required (such as the README.md, or the userdocs directory)
• Manually tested
• Made sure the title of the PR is a good description that can go into the release notes
• (Core team) Added labels for change area (e.g. area/nodegroup) and kind (e.g. kind/improvement)

BONUS POINTS checklist: complete for good vibes and maybe prizes?! 🤯

• Backfilled missing tests for code in same general area 🎉
• Refactored something and made the world a better place 🌟

[github-actions]

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