macOS: Fix crash on out-of-bounds text input ranges (#191780)

`TextInputModel` assumes its selection and composing range fall within
its text, but nothing in the text input plugin enforced that. When they
don't, we index past the end of `text_` which triggers an abort due to
out of range access.

Previously, `setMarkedText:selectedRange:replacementRange:` passed
`selectedRange` unvalidated. IMEs report `NSNotFound` to indicate no
selection within the mark text. That maps to `NSIntegerMax` rather than
an offset, and we incorrectly stored that as the selection. Clearing the
marked text collapses the composing range, so the next update replaces
the selection instead and calls `replace` with a position
`NSIntegerMax`. Clear and re-mark is what an input method emits when the
input source is switched mid-composition.

`SetText` was also unsafe: it assigned `text_` before validating the
ranges, so a rejected update left us with the new text but the old (now
out-of-bounds) ranges still pointing into it.

This fixes that by clamping both ranges before any operation that
indexes the text, applying either all of `SetText`'s fields or none, and
clamping `selectedRange` and `replacementRange` in the plugin.
`setMarkedText:` also null-checks the UTF-16 buffer, which
`cStringUsingEncoding:` can return as nullptr.

Fixes: https://github.com/flutter/flutter/issues/190704

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