Edit an existing assignment instead of delete-then-re-add (Fixes #57)

[juandresrodca]

Edit an existing assignment instead of delete-then-re-add (Fixes #57)

⚠️ Needs dev-tenant validation before submitting. This changes a Graph mutating path
(/assign). The target-matching logic is unit-tested, but the end-to-end Graph result should be
confirmed in a test tenant first (see Testing).

Problem (root cause)

AddAssignmentButton.ps1 fetches the policy's current assignments, appends the new assignment,
and re-POSTs the whole set to /assign. When the selected target is already assigned (e.g. an app
already assigned to All Devices), the set then contains two assignments for the same target.
Graph ignores the duplicate, so a newly added include filter never takes effect, and the user has to
delete the assignment and re-create it (issue #57).

Fix

Before appending the new assignment, drop any existing assignment that targets the same audience,
so the new selection replaces it. A small helper Test-SameAssignmentTarget matches:

• All Users / All Devices: virtual targets with no groupId, matched on @odata.type alone.
• group include / exclude: matched on @odata.type and groupId.

Net effect: re-adding an already-assigned target now updates it (adds/changes the filter, intent,
or settings) instead of being ignored, which is the "edit assignment" behaviour requested.

Autopilot profiles are unaffected (they POST individual assignments and don't go through this set).

Testing

• Unit-tested Test-SameAssignmentTarget (function extracted via AST): All Devices vs All Devices
  (match), All Devices vs All Users (no match), group g1 vs g1 (match), g1 vs g2 (no match), include g1
  vs exclude g1 (no match).
• End-to-end (simulated): existing [All Devices no-filter] + [Group g1], adding
  All Devices + filter gives [Group g1] + [All Devices WITH filter] (the filterless one is
  replaced).
• Parses with no new errors; no BOM/encoding changes.
• Not yet validated against a live tenant. Recommended before merge: pick an app already assigned
  to All Devices, use Add Assignment on All Devices with an include filter, save, and confirm the
  filter now shows (and no duplicate/again-ignored behaviour).

[juandresrodca]

Let me know if is require some change on the code.
Cheers