[Feature] Encapsulate debug config provider with controlled getter

[EmmaYuan1015]

Problem

The debugConfigProvider is exported as a global nullable variable, leading to:

• Null check dispersal: Every consumer must check for null before usage
• Lifecycle risk: Race conditions at extension activation/deactivation boundaries
• Type safety: Non-null assertions required despite nullable type union
• Maintenance burden: Hard to track all null checks across codebase

Related code:

• rn-extension.ts:62 - Global nullable export
• selectAndInsertDebugConfiguration.ts:25 - Consumer null check

Solution

Replace export global variable with controlled getter pattern:

• Private state: debugConfigProvider
• Public API: getDebugConfigProvider() - throws if not initialized
• Internal setter: setDebugConfigProvider() - used only during lifecycle

Benefits

✅ Centralized null validation (single point of check)
✅ Clear initialization contract (throws with descriptive error)
✅ Type-safe for consumers (non-null guarantee)
✅ Improved code maintainability (fewer defensive checks)
✅ Better lifecycle management

Changes

• src/extension/rn-extension.ts: Added getter/setter pattern
• src/extension/commands/selectAndInsertDebugConfiguration.ts: Use getter, remove null check

Testing

✅ Build passes: 0 errors, 161 warnings (pre-existing)
✅ TypeScript: No compilation errors
✅ Formatter: All Prettier checks passed