MI-315: Upgrade gadget deployment pipeline

[kai-nguyen-aligent]

Description of the proposed changes

• Replaced push-staging and deploy-production boolean inputs with a single action input — callers must now pass action: push or action: deploy instead of setting separate booleans.
• environment-name is no longer defaulted to "staging" — it is now optional overall but required when action: push, with a runtime validation step that fails fast if missing.
• Temporary environment strategy for production deploys — when action: deploy, the workflow automatically:
  • Creates a temporary Gadget environment (deploy-<run_id>)
  • Pushes the checked-out code to it
  • Promotes (deploys) to production
  • Cleans up the temporary environment (runs even on failure)
    This ensures only the exact code from the Git ref is promoted, avoiding unreviewed changes in shared staging.

Notes to reviewers

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[kai-nguyen-aligent]

Thanks, @TheOrangePuff I addressed the check for action in test step and add my comment/opinion for others. Could you please check and let me know your thought.

[TheOrangePuff]

LGTM!