Fix autocomplete double insertion by using live cursor position for replacement range

[NamanGoyalK]

This PR fixes a race condition in the editor where accepting an autocomplete suggestion would sometimes result in duplicated characters (e.g., typing Text.ri and selecting rich resulted in Text.richri).

fixes #3562

The Issue

The issue was caused by a "stale state" race condition:

1. When the completion request is sent (e.g., at Text.), the Analysis Server calculates a replacement range based on that snapshot (length 0).
2. If the user continues typing (e.g., ri) while the request is in flight, the cursor advances.
3. When the user accepts the suggestion, the editor applied the original (stale) replacement range, failing to consume the newly typed characters.

The Fix

This change updates _applyCompletion in editor_service.dart to calculate the replacement range dynamically at the moment of selection.

Instead of relying solely on the static replacementLength returned by the server, we now calculate the effectiveEnd of the replacement using the live cursor position:

final effectiveEnd = math.max(cursorIndex, serverEnd);

---

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