D-17: ThinkingIndicatorView — collapsed-by-default thinking block

[kirich1409]

Description

Implement the collapsible ThinkingIndicatorView that surfaces the agent's extended thinking blocks. Collapsed by default; expanding reveals raw monospaced text (not rendered as markdown — thinking is raw reasoning).

Spec: Epic #250 §6 (thinking block behavior); docs/architecture/dialogue-events.md §2 (thinking events), §6.5 (long thinking handling).

Scope

File

MacApp/Packages/AgentChatUI/Sources/AgentChatUI/Messages/ThinkingIndicatorView.swift

Structure

```swift
public struct ThinkingIndicatorView: View {
public init(thinking: Thinking, isStreaming: Bool)
}

public struct Thinking: Equatable, Sendable {
public let text: String // accumulated thinking content
public let tokenCountApprox: Int? // if available
public let elapsedSeconds: TimeInterval?
}
```

• Header (always visible): SF Symbol "brain" + Thinking… / Thinking (2s · ~420 tokens) + DS.Color.textTertiary text.
• Streaming indicator: spinner or ellipsis "…" next to "Thinking" when isStreaming == true. Guarded by reduceMotion.
• Collapsed by default; disclosure chevron on right.
• Expand → show Thinking.text in monospaced font, inset indent, DS.Color.textSecondary. No markdown rendering.
• After content_block_stop (streaming = false), header transitions to lightbulb 💡 icon (or SF Symbol lightbulb.fill) with final tokenCount.

A11y

• .accessibilityLabel: "Agent thinking block, collapsed/expanded, N tokens".
• Expanded content readable by VoiceOver (raw text).

Acceptance Criteria

• ThinkingIndicatorView implemented.
• Collapsed by default; click header expands / collapses with smooth animation (reduceMotion-guarded).
• Streaming state shown via spinner or ellipsis; transitions to static icon on completion.
• Token count + elapsed time displayed when available.
• Expanded content in monospaced font, no markdown.
• VoiceOver can expand/collapse and read content.
• Snapshot tests: collapsed, expanded, streaming, completed.
• DS tokens only.

Relationships

• Parent issue: EPIC: Dialogue — structured chat UI for agent sessions #250
• Blocked by: #D-12, D-2: DesignSystem — add StatusIndicator primitive #252 (StatusIndicator optional reuse)
• Blocks: #D-14 (AssistantMessageView integration), #D-18