[Feature] Passive exception capture mode in the Debugger tab

## Problem

The Debugger's existing exception modes (Ignore, Uncaught, All) - all share the same behaviour: the VM pauses and waits for the developer to resume. This breaks down for intermittent bugs, timing-sensitive flows, and exceptions that are hard to reproduce on demand. There is currently no way to let the app run freely and collect exception data without altering the conditions that produced it.

The Logging tab surfaces `FlutterError` output but as a raw unstructured stream - no deduplication, no route context, no export. It is not designed for this workflow.

## Proposed solution

Add a fourth option to the existing exception mode selector in the Debugger tab:
- Ignore exceptions
- Uncaught exceptions
- All exceptions
- Log exceptions <- new

When active, DevTools listens to the VM Service Debug event stream for exception events without sending a resume signal - the VM is already emitting these, this mode just doesn't pause on them. Captured exceptions are deduplicated by stack trace hash and written continuosly to `Example - ".devtools/exception_log_.md"` at project root via the DevTools local server process written to disk as exceptions are captured.

Each entry captures exception type, message, stack trace, current route, timestamp, and occurrence count for Debug Mode only.