test: make constructor rename regression assertions unconditional

[shunichironomura]

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Review finding: M01 · Priority: P3

crates/graphcal-lsp/src/rename.rs:893–918 wraps its checks in if let Ok(Some(result)) = rename(...). Returning an error or no edit makes the test pass. It also constructs and immediately discards a LineIndex (let _ = lines).

Require successful rename with an explicit assertion, check exactly the intended spans, apply the edit, and compile the result. Remove the unused index construction. This should be done with the #1580/#1581 regression work, not as an unrelated broad style refactor.

Acceptance criteria

• rename_constructor_edits_all_occurrences explicitly requires Ok(Some(edit)); an error or absent edit fails the test.
• Assert exactly the intended definition, initializer, and match-arm spans, not merely a lower bound on the edit count.
• Apply the edits and verify the resulting source checks successfully.
• Remove the unused LineIndex construction and discard.

Sequencing

Coordinate with #1580 and #1581 to strengthen the rename regression suite. This test cleanup is independently implementable and should not require a broad style refactor.

Review context

Identified during the 2026-09-06 codebase review at workspace commit 7f2944b3d0d3d5a95cf1c10b7ad1ba7bb80228a4.