feat: add Import-PiHoleTeleporter, completing API coverage

[mikemadeja]

Summary

• Adds Import-PiHoleTeleporter (POST /teleporter) - the last remaining functional gap in the Pi-hole v6 API, aside from two low-value API self-discovery endpoints (GET /endpoints, GET /docs) already agreed to be out of scope
• Restores a Teleporter archive (as produced by Get-PiHoleTeleporterDownload) with flat switch parameters (-Config, -DhcpLeases, -Group, -Adlist, -AdlistByGroup, -Domainlist, -DomainlistByGroup, -Client, -ClientByGroup) choosing what to restore, instead of mirroring Pi-hole's own nested request shape. Omitting every switch imports everything, matching the API's own default
• This overwrites the current configuration, so unlike every other function in the module it prompts for confirmation by default (ConfirmImpact = 'High') rather than only when -Confirm is passed explicitly - pass -Confirm:$false to skip the prompt, or -WhatIf for a true dry run (no request is ever sent)

Two things confirmed by hand against a real server that don't match the docs

• The response's processed-items array is actually returned under files, not processed as the OpenAPI spec documents - the function reads the real field
• Pi-hole reloads internally right after any import and briefly refuses connections for a few seconds afterward - both the integration tests and the docs example script settle with a short delay to avoid a flaky next call

Test plan

• Integration tests pass against a live server (5/5): real selective import (restoring the server's own just-exported backup - a no-op, safe to run repeatedly), -WhatIf performs no request, nonexistent file errors, non-ZIP file content errors (real 400 from the API), bad password errors
• Manually confirmed -WhatIf sends zero requests and a non-interactive call without -Confirm:$false correctly refuses to proceed rather than silently running
• Manually confirmed generating/restoring does not leave the module or server in a broken state - the transient post-import unavailability clears within a few seconds
• PSScriptAnalyzer clean
• Full non-integration unit suite passes (12/12)
• README.md / docs/EXAMPLES.md regenerated

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