[ice-pring]

MoveOperation accepts a JsonPointer object in from, but its public source-location helpers try to parse that object as a string. Reading from_path or from_key, or changing the source with set_from_part(), therefore raises TypeError even though the operation can be applied.

Reuse _from_pointer() in those three places, as apply() already does. Added regressions for plain and custom pointer classes, including editing the source and applying the resulting move.

Validation:

• The three added test methods produce six subtest errors before the fix; all eight source-location test methods pass afterward.
• Windows CPython 3.12.14 and 3.14.5: each full example/doctest suite runs 217 tests with one platform skip, and all 20 Hypothesis property tests pass.
• Ubuntu 24.04 / CPython 3.12.3: all 217 example/doctest tests and all 20 Hypothesis property tests pass, with no skips.
• Source line and branch coverage: 100%.
• Flake8 syntax/name checks and git diff --check pass.