feat- Unify thinking block title under a single "Thinking" label and change cancel timing

[xinyi-gong]

resolve #192

[Copilot]

Pull request overview

This PR updates the chat thinking block UI to consolidate the visible thinking status text around a single Thinking label, addressing rendering issues from #192.

Changes:

• Replaces separate in-progress/completed/cancelled thinking message keys with one thinking_title.
• Updates ThinkingBlock completion/cancellation behavior to stop changing the title per state.
• Adjusts ThinkingTurnWidget to call the simplified showCompleted() API.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
ThinkingTurnWidget.java	Uses the new no-arg completion API and still handles async thinking title generation.
ThinkingBlock.java	Updates thinking block state/title behavior and exposes title-setting helper.
messages.properties	Consolidates thinking title strings into one Thinking message.
Messages.java	Updates NLS fields to match the consolidated message key.

Comments suppressed due to low confidence (1)

com.microsoft.copilot.eclipse.ui/src/com/microsoft/copilot/eclipse/ui/chat/ThinkingTurnWidget.java:104

• A sealed block remains non-finalized until this async title response runs. During that window, onChatMessageCancelled() can still call showCancelled() because SEALED is not considered finalized, so pressing Stop after the thinking stream has ended can still mark the already-complete thinking block as cancelled.

          target.showCompleted();