fix: add webview heartbeat monitoring for gray screen detection and recovery

[roomote]

This PR attempts to address Issue #10737 by implementing a webview heartbeat monitoring system.

Problem

Users report that the Roo Code panel occasionally turns completely gray during long-running tasks with auto-approval enabled ("brrrr mode"). The panel becomes unresponsive while the extension continues working in the background.

Solution

Implements a ping-pong heartbeat system to detect when the webview becomes unresponsive and provide auto-recovery options:

• Heartbeat monitoring: Extension sends "ping" messages to webview every 5 seconds
• Webview response: Webview immediately responds with "pong" messages
• Unresponsive detection: After 3 missed heartbeats (15 seconds), webview is considered unresponsive
• Auto-recovery: Shows notification with "Reload Panel" option to recover

Changes

• Add ping/pong message types to ExtensionMessage and WebviewMessage
• Implement heartbeat sender and monitoring in ClineProvider
• Add automatic detection of unresponsive webviews after 3 missed heartbeats
• Show notification with option to reload webview on detection
• Handle pong messages in webviewMessageHandler
• Respond to ping messages in webview ExtensionStateContext

Testing

• All existing ClineProvider tests pass
• All ExtensionStateContext tests pass
• TypeScript compilation passes for both src and webview-ui

Feedback and guidance are welcome.

---

Important

Adds heartbeat monitoring to detect and recover from unresponsive webviews by sending periodic "ping" messages and expecting "pong" responses.

• Behavior:
  • Implements heartbeat monitoring in ClineProvider to detect unresponsive webviews by sending "ping" messages every 5 seconds and expecting "pong" responses.
  • After 3 missed "pong" responses (15 seconds), considers the webview unresponsive and shows a notification with a "Reload Panel" option.
• Message Handling:
  • Adds "ping" and "pong" message types to ExtensionMessage and WebviewMessage.
  • Handles "pong" messages in webviewMessageHandler to reset the heartbeat timer.
  • Responds to "ping" messages in ExtensionStateContextProvider by sending "pong" back.
• Integration:
  • Starts heartbeat monitoring in ClineProvider when a new session begins.
  • Stops monitoring when the provider is disposed.
  • Updates webviewMessageHandler to handle "pong" messages and reset the heartbeat timer.

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[roomote]

Rooviewer   See task on Roo Cloud

Code review complete. No issues identified.

The heartbeat monitoring implementation is well-structured:

• Type definitions for ping/pong messages are properly added
• Heartbeat interval and timeout logic is correctly implemented
• Edge cases are handled appropriately (hidden view, dispose cleanup)
• Recovery mechanism properly regenerates webview HTML and restarts monitoring

_{Mention @roomote in a comment to request specific changes to this pull request or fix all unresolved issues.}