fix: use VS Code commands for continue/step to fix exception UI

[TPreece101]

Summary

• Fix exception-after-continue bug: session.customRequest('continue') bypassed VS Code's internal debug model state updates, so when an exception occurred after continuing from a breakpoint, the UI never showed the exception and the debugger appeared stuck. Now uses vscode.commands.executeCommand('workbench.action.debug.continue') (and equivalent step commands) by default, which properly updates the model state. Falls back to customRequest only when explicit threadId or granularity is needed.
• Pass stop reason metadata to LLMs: Tool responses now include reason (e.g. "breakpoint", "exception", "step"), description, and exceptionText fields so LLMs can distinguish why the debugger stopped and see exception messages without needing a separate evaluate call.
• Add regression test: New exception-continue.test.ts suite covers the breakpoint → continue → exception scenario, verifying proper stop detection and that exception metadata is present in the response.

Test plan

• All 77 tests pass (npm test), including new exception-after-continue suite
• Manual: set breakpoint, launch, continue via MCP, trigger exception — verify exception shows in VS Code UI
• Manual: verify LLM-facing response includes reason: "exception" and description fields

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 87.80488% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.70%. Comparing base (52f7379) to head (e34c467).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/debug-server.ts	85.91%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #21      +/-   ##
==========================================
+ Coverage   74.87%   75.70%   +0.83%     
==========================================
  Files           7        8       +1     
  Lines        1210     1268      +58     
  Branches      152      160       +8     
==========================================
+ Hits          906      960      +54     
- Misses        302      306       +4     
  Partials        2        2              

Flag	Coverage Δ
extension	75.70% <87.80%> (+0.83%)	⬆️

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