fix: 3.8 Update Redfish boot config for iDRAC10 (#477)

[goldberl]

Description

Modern iDRAC rejects PATCH requests to the
redfish/v1/Systems/{node_id} endpoint. Instead it expects PATCH requests to redfish/v1/Systems/{node_id}/Settings.

This PR implements dynamic endpoint discovery:

• Extracts the boot configuration endpoint dynamically by querying the main System object payload and extracting the @Redfish.Settings URI (SettingsObject -> @odata.id).
• Falls back to the standard legacy path if the property is missing or if the payload fetch fails.
• Refactors get_etag to accept a target endpoint path.
• Adds helper method get_boot_endpoint for set_pxe_boot to use.

Resolves: LP:2160731

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(cherry picked from commit 902480d)

[github-actions]

[maas-code-reviewer review]

LLM-generated review from https://github.com/canonical/maas-code-reviewer.
Intended to assist a human reviewer, not replace one — suggestions may be
incorrect, please verify before acting.

The implementation correctly and defensively identifies the Redfish Settings endpoint with a robust fallback mechanism. The test coverage is comprehensive and accurately verifies all the new dynamic endpoint discovery paths.

There is a minor suggestion inline to simplify a wrapper function, but otherwise, this change looks solid and cleanly resolves the issue with modern iDRACs.

[AloizioMacedo]

Let's avoid merging to 3.8 for now due to the current release process of 3.8.0, which is going through several test runs with multiple points of failure, including external ones like changes in LXD.

I will unblock and merge this at a later point. Thanks, @goldberl !