feat: add Config write/property functions, completing the Config API area

[mikemadeja]

Summary

• Extends Get-PiHoleConfig with -Element (request one subset of the config tree, e.g. "dns/upstreams") and -Detailed.
• Adds Set-PiHoleConfig (PATCH /config), Add-PiHoleConfigArrayItem/Remove-PiHoleConfigArrayItem (PUT/DELETE /config/{element}/{value}, for array-type settings like dns/hosts and dns/cnameRecords - this is where local DNS records and CNAMEs live), and Get-PiHoleConfigProperty (the list of settings that can never be changed via the API).
• All three write functions require the app password's app_sudo flag to be enabled in Pi-hole (Settings > All Settings) - Pi-hole blocks config changes from app passwords by default, and there's no way to enable it via the API itself (that would defeat the point of the restriction). This is documented in each function's .DESCRIPTION.
• Adds ConvertTo-PiHoleFriendlyErrorMessage (Private/Misc.ps1), used by the three write functions' error handling: Pi-hole's own error responses carry a clearer message/hint than the generic HTTP exception text, and for the app_sudo case specifically, a concrete pointer to where to enable it - previously the module surfaced only "403 (Forbidden)" or "400 (Bad Request)" with no explanation of why or how to fix it.

This completes the "Config" area from the API coverage audit (5 of 5 operations implemented).

Test plan

• All behavior verified directly against the real Pi-hole server, both with app_sudo disabled (confirming the 403 and its improved error message) and enabled (confirming a real config value can be changed and reverted, an array item can be added and removed, and a genuinely read-only property (misc.readOnly) is correctly rejected with a clear reason).
• 18 new/updated Pester integration tests across the 5 functions, all passing against the real server. Write-path tests change and restore only a harmless debug flag (debug.api) or a fake TEST-NET-1 host entry - never real DNS/DHCP/network behavior.
• Confirmed server config fully restored to its original state after every test run and after regenerating docs/EXAMPLES.md.
• Invoke-ScriptAnalyzer clean against the full module.
• Full non-integration Pester unit suite passes (4/4; unaffected by this change).

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