fix: preserve DevTunnel error message when HostAsync fails

[PureWeen]

Problem

When clicking "Start Dev Tunnels", the button briefly flashes a spinner then returns to its initial state with no error message — no QR code appears and no explanation is shown.

Root Cause

In DevTunnelService.HostAsync(), when TryHostTunnelAsync() fails (both initial and retry attempts), Stop() is called for cleanup at line 237. Stop() transitions through Stopping → NotStarted, and SetState(NotStarted) clears _errorMessage (since only the Error state preserves it). The error that was set by SetError() during the failed attempt is wiped before the UI can display it.

The UI then shows the start button again (state = NotStarted) instead of the error panel (state = Error).

Fix

1. Preserve error across Stop(): Save _errorMessage before Stop(), then re-set it via SetError() after cleanup so the user sees what went wrong.
2. Catch block cleanup: Call Stop() before SetError() in the catch block so the WsBridge started earlier in the method is properly cleaned up on unexpected exceptions.

Tests Added

• Stop_ClearsErrorMessage_ByDesign — confirms Stop() clears error (documents the root cause behavior)
• HostAsync_WhenTunnelFails_PreservesErrorMessage — end-to-end: verifies failed HostAsync ends in Error state with a message
• ErrorPreservation_SaveAndRestore_AcrossStop — tests the save/restore pattern used in the fix
• SetError_SetsStateToError_AndPreservesMessage — verifies SetError behavior
• SetError_FiresOnStateChanged — verifies event fires

All 85 DevTunnel tests pass. All 1970 passing tests remain green (13 pre-existing PopupThemeTests failures unrelated).

[PureWeen]

PR Review Squad — PR #283 Round 1

CI Status: ⚠️ No checks reported

Summary

Good fix — preserves DevTunnel error message across Stop() call using save/restore pattern. Catch block correctly adds Stop() before SetError().

🟡 MODERATE

1. DevTunnelServiceTests.cs:376-398 (5/5) — HostAsync_WhenTunnelFails_PreservesErrorMessage is vacuous: all assertions are inside if (!result). If devtunnel CLI is installed in the test environment, HostAsync returns true, the if block is skipped, and the test passes without validating anything. Add Assert.False(result) unconditionally or use a mock/stub.

2. DevTunnelService.cs:237-241 (3/5) — if (!string.IsNullOrEmpty(lastError)) skips error restore if TryHostTunnelAsync returned false without calling SetError. Service lands in NotStarted with no error — the original bug. Fix: SetError(lastError ?? "DevTunnel failed to start").

🟢 MINOR

• DevTunnelService.cs:235 (5/5) — _errorMessage race with Exited handler (tiny window, low risk)
• DevTunnelService.cs:237 (2/5) — Triple OnStateChanged notification (Stop + SetError)

Recommended Action: ⚠️ Request changes

Two specific asks: (1) make the test non-vacuous, (2) add fallback error message for null lastError.