agentserver: use FOUNDRY_HOSTED for hosted environment detection

[ammaaradammicrosoft]

Replace the heuristic three-var + FOUNDRY_ENVIRONMENT check with a simple opt-in: is_hosted returns True only when FOUNDRY_HOSTED=true, which the platform injects exclusively into hosted containers.

This prevents false-positive Foundry storage auto-activation when a developer sets FOUNDRY_PROJECT_ENDPOINT or other FOUNDRY_* vars locally (e.g. to use agent_framework features) without intending to run in a hosted environment.

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