fix(flterm): recover text input when a delta desyncs from the sentinel

[jacobaraujo7]

Summary

On iOS, composing an accent (dead key + vowel) freezes the terminal: after the
accent commits, no further typing registers. Plain keys are fine until the first
accent.

Root cause

TextInputSession keeps a one-space sentinel and pushes it back via
setEditingState after every commit, translating deltas relative to it. iOS,
however, keeps its own marked-text buffer and ignores that reset right after
a composition. An accent commits an IME-like character (> 0x7f), the buffer is
reset on our side but not on iOS's, and the next delta iOS sends references
offsets past our sentinel value.

delta.apply(_value) then throws a RangeError. Because that happens inside
updateEditingValueWithDeltas, the exception tears down the platform text-input
channel — every subsequent keystroke is dropped, so typing freezes entirely.
Desktop platforms honor the reset, so they never hit it.

Fix

Guard each delta.apply; on failure, resync the platform back to the sentinel
and drop that batch instead of letting the exception kill the connection. The
channel stays alive and input keeps working.

Test

Adds a regression test that drives a delta whose range exceeds the sentinel
(which throws today) and asserts the session recovers, stays attached, and keeps
committing subsequent input. flutter analyze is clean; the full flterm suite
passes.

Note: the throwing delta is reproduced synthetically in the test. Validation
of the exact iOS accent sequence on a device/simulator is still recommended,
but the recovery is a strict robustness improvement regardless of the trigger.

[elias8]

The tests pass with and without the fix. I'm also not able to reproduce this on my phone; have you? What are the steps?