refactor(agent): replace metadata.is_case_manager with metadata.variant

[dianagrecu-uipath]

What changed

• Added AgentVariant enum (CaseInsensitiveEnum) with CASE_MANAGER = "caseManager"
• Replaced is_case_manager: bool field in AgentMetadata with variant: Optional[AgentVariant] = None
• Updated the AgentDefinition.is_case_manager property to derive from metadata.variant — public API unchanged, still returns bool
• Updated test fixtures from isCaseManager: true/false to variant: "caseManager" / absent

Notes

• variant is Optional[AgentVariant] because regular agents have no variant set. There is no "no variant" enum value - that state is None. The boolean is_case_manager property hides this from SDK consumers.