Fix forward word deletion at the end of a text input

[rupayon123]

Problem

In textinput, Alt+D panics when the cursor is on the final rune of the value. When the cursor is on the final rune of a word followed by another word, it also deletes that next word (#1056).

Change

Scan from the current rune using a separate end index. Only skip whitespace when deletion starts on whitespace; then stop at the next word boundary. This avoids indexing beyond the value and preserves the existing behavior for deletion starting on a space. The new Update-driven regression covers the reported cases and a Unicode rune.

Validation

• New regression failed before the fix with the extra-word deletion and index panic; passed afterward.
• go test ./textinput -count=1 — passed.
• go test ./... — passed.
• go test -race ./textinput -count=1 — passed.
• go vet ./..., gofmt -d on changed files, and git diff --check — passed.

Tested with Go 1.27.1 against the module's Go 1.25 baseline. Prepared with AI assistance.

Fixes #1056.