Add environment parameter support

[AngeloGiacco]

Summary

• Add optional environment parameter to ConversationConfig (defaults to null, server defaults to "production")
• Pass environment as a query parameter to the GET /v1/convai/conversation/token endpoint
• Add fetchSignedUrl() method to TokenService with environment support for GET /v1/convai/conversation/get_signed_url
• Add SignedUrlResponse data class

Test plan

• Verify existing tests pass (no breaking changes — environment defaults to null everywhere)
• Test public agent token fetch with environment set (e.g. "staging")
• Test public agent token fetch without environment (default behavior unchanged)
• Test fetchSignedUrl with and without environment param

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