Allow adding regions to existing namespaces

[adityavad]

Overview

Allow adding regions to an existing namespace via Terraform, enabling multi-region namespaces (MRN) without recreating the namespace. Region removals and replacements remain blocked for safety.

Closes #401

Changes

• Replace areRegionsEqual (which blocked all region changes) with classifyRegionChanges, which distinguishes additions from removals — additions are allowed, removals/replacements are blocked
• Extract region update logic into a testable updateNamespaceWithRegions function behind a namespaceRegionUpdateClient interface
• Use AddNamespaceRegion for region additions instead of passing planned region order through UpdateNamespace
• Preserve current region order in UpdateNamespace calls to avoid accidental failover from list reordering
• Handle InvalidArgument "nothing to change" from UpdateNamespace when the only change is a region addition — skip to AddNamespaceRegion instead of failing
• Add schema-level region count validation (SizeBetween(1, 2))

Testing

• Unit tests for classifyRegionChanges (6 cases: no-change, reorder, addition, prepend, replacement, removal)
• Unit tests for updateNamespaceWithRegions using a fake client (region addition sequence, removal blocking before API calls, "nothing to change" skip-to-add scenario)
• Tested on our local terraform temporal repo:

Before change:

After change:

Checklist

• This code has been self-reviewed
• Tests were added/updated as appropriate

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