fix(web): restore backspace-input token reconstruction 🚂

[jahorton]

Before epic/autocorrect, input of a pure backspacing transform would perform special operations within the predictive-text worker, reconstructing the remainder of the token and erasing fat-finger data. This PR fixes the regression.

This is notably useful when attempting to erase part of an applied suggestion, as applied suggestions wholesale-replace the original versions of affected tokens with a single transform input. Backspacing into the transform will then break that unitary transform into pieces better suited for ongoing predictive-text operations.

Build-bot: skip build:web build:android

User Testing

TEST_SUGGESTION_ERASURE: Using Keyman for Android...

• apply a suggestion for a long word
• progressively erase it with backspace inputs
• verify that reasonable suggestions are displayed after each backspace.

TEST_SMOKE: Using Keyman for Android, play around with a keyboard-model pair supporting predictive text for two minutes. If any odd, unusual, or unexpected behaviors occur, fail this test and report them here.

• Do not use a Khmer-script keyboard for this, as there are separate known issues for such keyboards.

[keymanapp-test-bot]

User Test Results

Test specification and instructions

• ✅ TEST_SUGGESTION_ERASURE (PASSED) (notes)
• ✅ TEST_SMOKE (PASSED) (notes)

Test Artifacts

• Android
  • KeyboardHarness apk - build : all tests passed (no artifacts on BuildLevel "build")
  • KMSample1 apk - build : all tests passed (no artifacts on BuildLevel "build")
  • KMSample2 apk - build : all tests passed (no artifacts on BuildLevel "build")
• Web
  • KeymanWeb Test Home - build : all tests passed (no artifacts on BuildLevel "build")

[Meng-Heng]

Test Specs

1. Android 15.0 Pixel 9a API 35
2. Keyman for Android 19.0.252-alpha-test-16290

Test Results

• TEST_SUGGESTION_ERASURE (PASSED):

1. Launch Keyman and Dismiss the "Get Started" screen
2. Apply And and and from the default suggestions
3. Text result: And and
4. Type sug, Apply suggestions
5. Text result: And and suggestions
6. Continue typing with approx, Apply approximately
7. Text result: And and suggestions approximately
8. Backspace once
9. Text result: And and suggestions approximately
   • Predictions: approximately
10. Keep backspacing until reaching the typed text of And and suggestions appro
    • Predictions: "approach appropriate approved approval" ...etc.
11. Backspace once, text result And and suggestions appr
    • Predictions: "appreciate approach appropriate approved" ...etc.
12. Backspace once, text result And and suggestions app
    • Predictions: "app apparently appreciate apply" ...etc.
13. Backspace once, text result And and suggestions ap
    • Predictions: "app apparently appreciate apply" ...etc.
14. Backspace once, text result And and suggestions a
    • Predictions: "and are as at" ...etc.
15. Backspace once, text result And and suggestions
    • Predictions: "and are as at" ...etc.
16. Backspace once, text result And and suggestions
    • Predictions: suggestions
17. Backspace once, text result And and suggestion
    • Predictions: "suggestion suggestions"
18. Backspace twice, text result And and suggesti
    • Predictions: "suggestions suggestion suggesting" ...etc.
19. VERIFIED: The suggestions are working to predict the current typed text.

• TEST_SMOKE (PASSED):

1. Continue to the next line after TEST_SUGGESTION_ERASURE
2. Type percisel (typing in correctly)
3. Predictions: precisely
4. Apply suggestion
5. Text result precisely
6. Backspace one at a time over the word precisely
7. The word suggested are acceptable
8. Retype and Reapply precisely
9. Insert the next line
10. Apply the and the from the default suggestions
11. Type prett
12. Apply prettier
13. Backspace over each character of prettier
14. VERIFIED: The prediction works correctly.