[Hubs] Make upgrades and redeploys from the portal keep existing options #2397

📝 Scenario

As a FinOps hub admin, I need to upgrade or redeploy my hub from the Azure portal without re-entering every option in order to avoid options resetting to defaults or the deployment failing on options that can't change after creation.

⚠️ Problem

The portal path (createUiDefinition) starts from defaults on every deployment. Admins must remember and re-set every option each time. Options not set again reset to defaults, and some create-only options fail the deployment if they differ from the existing hub. Retention is the only area that already keeps current values when left at the default (#2206). Unknown whether this can be fixed in the toolkit alone or needs portal/ARM support, so the scope below starts with what we control.

💎 Solution

• Docs: recommend a parameter file for every upgrade/redeploy (upgrade guide and deploy guide), with how to create and upload one.
• PowerShell: add a command that reads the existing hub's deployment options, writes a parameter file, then deploys with it. Proposed name: Export-FinOpsHubParameter (reads and writes the file) plus a -ParameterFile option on Deploy-FinOpsHub; or one command, Update-FinOpsHub, that does all three steps. Pick one during spec; both use approved verbs.
• Investigate whether the portal path can prefill from the existing deployment; if not, say so in the upgrade docs.

📋 Tasks

• Docs: parameter-file upgrade steps in the upgrade guide and deploy guide
• PowerShell: command to read existing options, create a parameter file, and deploy with it (with unit tests and docs-mslearn page)
• Decide and document which options can't change after creation
• Changelog entry

✅ Acceptance

• A user can upgrade an existing hub to the latest version, keeping every option, without typing any option twice.
• Docs state clearly which upgrade paths keep current options and which don't.

ℹ️ Additional context

Related: #2206 (upgrade lost retention). Raised in PR #2394 review.

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