feat(debugger): add expression evaluation and variable editing tools

[CalvinAllen]

Summary

• Adds debugger_evaluate tool for evaluating arbitrary expressions in the current debug context (like the Immediate Window), using Debugger2.GetExpression()
• Adds debugger_set_variable tool for modifying local variable values during a debug session, using Expression.Value setter
• Adds ExpressionResult model for returning evaluation results with expression, value, type, and validity
• Both tools require the debugger to be in Break mode

Closes #47

Test plan

• Break at a breakpoint, use debugger_evaluate with a simple variable name — should return its value and type
• Use debugger_evaluate with a complex expression (e.g., list.Count, x + y) — should evaluate correctly
• Use debugger_evaluate with an invalid expression — should return IsValidValue: false
• Call debugger_evaluate when not in Break mode — should return appropriate message
• Use debugger_set_variable to change a local variable's value, then debugger_get_locals to verify the change
• Use debugger_set_variable with a non-existent variable name — should return failure
• Use debugger_set_variable with an invalid value for the type — should return failure