Resolve the effective Static Web Apps environment in deployment preview

[vhvb1989]

Follow-up from #10204.

The Static Web Apps deployment preview currently reports the production environment. The actual deployment can omit the environment argument when swa-cli.config.json is used, allowing that configuration to select another environment.

Acceptance criteria

• Determine the effective Static Web Apps deployment environment from the applicable configuration when possible.
• Do not claim production when the environment cannot be determined without packaging.
• Keep preview read-only.
• Add coverage for the default production path, an explicitly configured environment, and an unknown environment.

Source review: #10204 (comment)