fix(textinput): cap suggestion completion to input width

[timatigoogl3-code]

Fix #812.

When suggestions are shown, the rendered completion tail is not bounded by the input width. With a value of `hel` and a long suggestion, the line rendered 69 visible cells with a width of 20, overflowing the space given to the textinput.

[timatigoogl3-code]

Notes for the reviewer:

• Root cause: completionView() rendered the whole suggestion tail without bounding it to the input width, and the padding block did not account for an already rendered completion.
• Fix: completionView(offset, maxWidth) truncates the tail via ansi.Truncate; padding is now width+1 - valueWidth - cursor - completionWidth.
• Regression test: Test_ViewRespectsWidthWithSuggestions. Without the fix it renders 69 cells at width 20; with the fix it renders 23 (prompt 2 + width 20 + cursor 1).
• Ran go test ./... — all green; gofmt/vet clean on the changed files.

(Changes authored with AI assistance; happy to adjust to the project's conventions.)