test: write and source_edit agree on edit validation #458

Parent: #451

Problem

source_edit skipped write's non-leaf rejection and produced a corrupting edit range (#435). Sibling paths that edit the same target must accept and reject the same inputs.

Scope

Add tests asserting that, for the same target and input, write and source_edit reach the same accept/reject outcome, including non-leaf targets and the other rejection cases write enforces.

Non-goals

Changing edit behavior beyond fixing any disagreement the tests expose. Other sibling pairs are tracked separately.

Acceptance criteria

• For every input write rejects, source_edit rejects it with an equivalent diagnostic and produces no edit range.
• For every input write accepts, source_edit accepts it.
• The test fails if either path drops a check the other enforces.
• Any disagreement found is fixed by sharing the established check, not by duplicating it.