Hide unsupported regions for hosted agents

[therealjohn]

Goal

Only deploy hosted agents to regions that support it so there are no downstream deployment errors.

Context

Hosted agents have limited availability in specific regions.
During azd ai agent init a developer is prompted with a list of all Azure regions, so its possible to select one that doesn't support hosted agents.

Current Behavior

Select location shows a full list of Azure regions. If you pick an unsupported one, you'll end up with a deployment error that the requested feature is not available in your subscription - which is also misleading since its related to region. The error message comes from the backend service.

Desired Behavior

Only deploy hosted agents to supported regions.

Implementation Notes

There's no API to get this information, we'd need to hardcode it or come up with our own dynamic solution for the list.

Also worth noting that this impacts all services in azd orchestration. There's no way to only deploy the hosted agent to a supported region and then deploy other resources to a different region without manually intervening with the Bicep, etc.

For example, it could be desirable that storage, or models be in a specific region due to governance concerns.

Acceptance Criteria

• Unit tests added or updated
• Existing test suite passes
• Lint or format passes

How to Test

azd ai agent init and select a sample.
Select to create a new foundry project
Select a subscription
Select from a subset of regions where hosted agents are available

[JeffreyCA]

We already have a hardcoded list of supported regions in the extension, but I recall @trangevi mentioning the list of supported regions for vNext may be different which we'll need to update:

azure-dev/cli/azd/extensions/azure.ai.agents/internal/cmd/init_locations.go

Lines 8 to 38 in 199a887

	// No API available to query supported regions for hosted agents, so keep hardcoded list based on public documentation:
	// https://learn.microsoft.com/azure/foundry/agents/concepts/hosted-agents#region-availability
	// TODO: List of supported regions for vNext may be different
	var supportedHostedAgentRegions = []string{
	"australiaeast",
	"brazilsouth",
	"canadacentral",
	"canadaeast",
	"eastus",
	"eastus2",
	"francecentral",
	"germanywestcentral",
	"italynorth",
	"japaneast",
	"koreacentral",
	"northcentralus",
	"norwayeast",
	"polandcentral",
	"southafricanorth",
	"southcentralus",
	"southeastasia",
	"southindia",
	"spaincentral",
	"swedencentral",
	"switzerlandnorth",
	"uaenorth",
	"uksouth",
	"westus",
	"westus3",
	}