fix(chassis): discover the default chassis member dynamically

[xibolun]

Problem

The SDK defaults chassis APIs to identifier 1 and constructs /redfish/v1/Chassis/1. Some compliant BMCs expose a different member, for example /redfish/v1/Chassis/Self.

Expected behavior

When chassis_id is omitted, discover the default chassis from the RootService.Chassis collection member link. Preserve explicit identifiers supplied by callers, and construct derived paths from the discovered chassis @odata.id.

Acceptance criteria

• A sole /redfish/v1/Chassis/Self member works with get_chassis() and chassis-derived APIs.
• Runtime SDK paths do not hard-code /redfish/v1/Chassis/1.
• Explicit caller-supplied chassis IDs remain supported.
• Unit tests cover dynamic Self discovery.