ci: don't run the full test suite for pure export additions to PiHoleShell.psm1

[mikemadeja]

Summary

• Every feature PR adds its new functions' names to PiHoleShell.psm1's Export-ModuleMember list - which previously always forced the full Pester suite to run on a PR build (via the "shared/core file changed" fallback). Since that's the one file nearly every PR touches, it was quietly defeating the changed-file scoping added in an earlier PR (chore: reorg tests to mirror module structure, scope CI to changed files #110): every recent feature PR ran the full suite regardless of how small its actual change was.
• A pure addition to the export list can't break any existing, already-passing test, so this only falls back to the full suite when an existing export was actually removed.
• Comparing whole diff lines doesn't work here since several exports share one line - adding a new function to an existing category's line shows as that whole line being removed and re-added, even though nothing already there was dropped. Instead, this extracts the quoted names from just the removed lines and just the added lines and compares those sets: a name that reappears in the added lines was only reformatted, not actually removed.
• PiHoleShell.psd1, PiHoleShell/Private/, and the other existing "always run everything" triggers are untouched - this only changes the .psm1 export-list case specifically.

Test plan

• Extracted the pipeline's PowerShell step in isolation and syntax-checked it.
• Verified against real PR diffs from this session using a git worktree (without touching the live pipeline or pushing anything): a pure-addition change (adding Config's new functions to the Config category's existing export line) correctly skips the fallback and reports "no full-suite fallback needed"; a synthetic removal case correctly still triggers the fallback.
• Note: PR feat: add Config write/property functions, completing the Config API area #123 (currently open) also touches PiHoleShell/Private/Misc.ps1, which is a separate, unrelated, intentionally-still-conservative trigger - so that PR will still run the full suite even after this change, exactly as expected.

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