fix: implement TextInputClient.onFocusReceived for Flutter >= 3.44 compatibility

[al-the-bear]

Problem

On Flutter 3.44+, appflowy_editor fails to compile with:

Missing concrete implementation of 'TextInputClient.onFocusReceived'.

Flutter 3.44 added TextInputClient.onFocusReceived() — a concrete member with a => false default on the TextInputClient mixin (packages/flutter/lib/src/services/text_input.dart). DeltaTextInputService mixes in DeltaTextInputClient, which only implements TextInputClient; the framework's default body does not carry over to an implements relationship, so the concrete class no longer satisfies the interface.

Fix

Add the onFocusReceived override to DeltaTextInputService, matching the framework default (=> false). It is a no-op on older Flutter versions where the member is unused, and restores compilation on 3.44+.

@override
bool onFocusReceived() => false;

Notes

• Verified against Flutter 3.44.6 (stable).
• Single-file, backward-compatible change; no behavior change on Flutter < 3.44.

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