fix: preserve chat cancel state during active turns

[jdneo]

fix #99

[Copilot]

Pull request overview

This PR fixes the chat action button state so Cancel/Stop remains available throughout a top-level chat turn, even when nested sub-agent turns complete, addressing issue #99 (“Missing stop button”).

Changes:

• Refactors ActionBar into an explicit state machine (ActionButtonState) to preserve Send/Cancel behavior across UI rebuilds and input changes.
• Updates ChatView to finish the action button state only when a top-level turn ends (not when a sub-agent turn ends).
• Adds unit coverage for the state transition logic and extends SWTBot test plan coverage for the regression scenario.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
CONTEXT.md	Documents terminology for top-level vs sub-agent turn lifecycles.
com.microsoft.copilot.eclipse.ui/src/com/microsoft/copilot/eclipse/ui/chat/ChatView.java	Ensures UI “turn finished” actions occur only on top-level turn completion.
com.microsoft.copilot.eclipse.ui/src/com/microsoft/copilot/eclipse/ui/chat/ActionBar.java	Introduces action-button state machine and rendering based on state.
com.microsoft.copilot.eclipse.ui.test/src/com/microsoft/copilot/eclipse/ui/chat/ActionBarActionButtonStateTest.java	Adds unit tests validating state transitions and invariants.
com.microsoft.copilot.eclipse.swtbot.test/test-plans/subagent/subagent.md	Adds regression test case validating Cancel remains available after sub-agent completion.
ACTION_BAR_LAYOUT_OPTIMIZATION.md	Captures deferred follow-up work for reducing control recreation/layout churn.