RSS handoff error message drops important cause information

My a4x2 is failing to set up, with only this log message:

18:01:04.828Z INFO SledAgent (RSS): Failed to handoff to nexus: Communication Error: error sending request for url (http://[fd00:1122:3344:101::6]:12221/racks/a6417c30-1748-45c8-b01e-caca9e11ec6c/initialization-complete)
    file = sled-agent/src/rack_setup/service.rs:1001

the full bunyan entry:

{"msg":"Failed to handoff to nexus: Communication Error: error sending request for url (http://[fd00:1122:3344:101::6]:12221/racks/a6417c30-1748-45c8-b01e-caca9e11ec6c/initialization-complete)","v":0,"name":"SledAgent","level":30,"time":"2025-05-27T18:01:04.828484117Z","hostname":"g0","pid":1312,"component":"RSS","file":"sled-agent/src/rack_setup/service.rs:1001"}

This error ought to have more information about what happened, but the cause is being dropped because we're just formatting the error as a string:
https://github.com/oxidecomputer/omicron/blob/37a2122e4be0d947c141833a695bf11a1c32f537/sled-agent/src/rack_setup/service.rs#L1001

We need to be using https://github.com/oxidecomputer/slog-error-chain here instead.

Read sled-agent/src/rack_setup/service.rs around line 1001 and inspect how the handoff error is formatted. Review the slog-error-chain usage linked in the issue, then verify that the RSS handoff log preserves the underlying cause information. Done means the logged error includes the relevant error chain rather than only the formatted top-level message.