[LIU-JUNYANG]

Summary

• Leave Enter and Shift+Enter unhandled by the agent composer's keyboard shortcut while the text controller has an active, non-collapsed IME composing range.
• Preserve ordinary Enter-to-send and Shift+Enter-to-insert-a-newline after composition ends.
• Add widget regressions for composing Enter, composing Shift+Enter, Enter after committing the text, and a collapsed composing range.

Problem

A user reported that typing English with a Chinese input method on macOS and pressing Return to confirm the text also sends the message. The report is from 3.0.0+49; the same keyboard handler is present on current master (3.0.0+50).

AgentComposer._handleKeyEvent previously treated every Enter key-down without Shift as a submit, without checking the text controller's composing state. The new check runs before either submitting or inserting a newline, so the composer does not consume Enter while composition is active.

Validation

• The new composing-Enter regression failed before the fix: expected zero submissions, observed one.
• dart format --output=none --set-exit-if-changed lib/agent/agent_composer.dart test/agent_composer_test.dart: passed.
• flutter test --no-pub test/agent_composer_test.dart: 17 passed.
• flutter test --no-pub --reporter expanded: 1,077 passed, 1 skipped, exit 0.
• flutter analyze --no-pub: exit 1 with three existing deprecated_member_use info diagnostics in lib/data/local/app_database.g.dart:15723 and lib/servers/maidcafe_push.dart:300,345; none in the changed files.
• git diff --check: passed.

The widget tests simulate IME composing state and keyboard events. The patched application has not been manually tested with a native macOS Chinese input method, so native IME event ordering and the original GUI reproduction still need confirmation. This change is limited to the composer's keyboard handler; it does not change the text-input action callback.

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