feat: add Update-PiHoleList, completing the Lists API area

[mikemadeja]

Summary

Implements PUT /api/lists/{list}, the last missing piece of List management - Add-PiHoleList's own "already exists" error has referenced this function by name since it was written. Updates an existing list's Comment, Group(s), and/or Enabled state.

Like Update-PiHoleGroup, the underlying API replaces the entire list on update, so this reads the list's current values first and only overrides whichever of Comment/Group/Enabled was actually passed - using the same $PSBoundParameters.ContainsKey() pattern (rather than checking parameter values for $null), since that was the actual root cause of the bug fixed in Update-PiHoleGroup: an unbound [string] binds to "", and [bool] can never be $null, so value-based checks can't reliably detect "not passed".

Verified type only needs to be a query parameter (?type=block), not also in the request body, by testing directly against the real server - the API's own documented PUT body schema lists type as a body field too, but (like the POST endpoint's documented schema, which turned out to expect type in the query string only despite listing it in the body schema) the server accepted both a query-only and query+body request identically.

The other remaining Lists gap, DELETE /lists/{list} (single item), isn't being added as a separate function: Remove-PiHoleList already achieves the same outcome via the batch endpoint (a batch of one) - same reasoning already applied to skip the deprecated POST /action/flush/arp.

With this merged, Lists is fully covered.

Test plan

• Invoke-ScriptAnalyzer -Path .\PiHoleShell -Recurse - clean
• Verified against a real Pi-hole v6 server: partial updates (confirming the untouched field is preserved), -RawOutput, and not-found/missing-parameter/bad-password errors
• New integration test file, 6 tests, all passing (re-ran after an initial transient failure caused by the real test server - a Raspberry Pi Zero W under this session's cumulative load - being unusually slow; confirmed not a code bug by re-running clean)
• Full non-integration suite passes with no regressions (149 tests)
• Regenerated README.md

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