test: Add comprehensive tests for platform and utils packages

Added extensive test coverage for previously untested files:

Platform package (94.69% coverage, +47 tests):
- paths.test.ts: 22 tests for path construction and directory creation
- security.test.ts: 25 tests for path validation and security

Utils package (94.3% coverage, +109 tests):
- logger.test.ts: 23 tests for logging with levels
- fs-utils.test.ts: 20 tests for safe file operations
- conversation-utils.test.ts: 24 tests for message formatting
- image-handler.test.ts: 25 tests for image processing
- prompt-builder.test.ts: 17 tests for prompt construction

Coverage improvements:
- Platform: 63.71% → 94.69% stmts, 40% → 97.14% funcs
- Utils: 19.51% → 94.3% stmts, 18.51% → 100% funcs

Updated thresholds to enforce high quality:
- Platform: 90% lines/stmts, 95% funcs, 75% branches
- Utils: 90% lines/stmts, 95% funcs, 85% branches

Total new tests: 156 (platform: 47, utils: 109)
All tests passing with new coverage thresholds.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

libs/utils/vitest.config.ts

@@ -11,12 +11,12 @@ export default defineConfig({
       include: ["src/**/*.ts"],
       exclude: ["src/**/*.d.ts", "src/index.ts"],
       thresholds: {
-        // Current overall coverage: ~19% (only error-handler.ts tested)
-        // Set modest thresholds until more files are tested
-        lines: 15,
-        functions: 15,
-        branches: 25,
-        statements: 15,
+        // Excellent coverage: 94.3% stmts, 89.77% branches, 100% funcs, 94.21% lines
+        // All files now have comprehensive tests
+        lines: 90,
+        functions: 95,
+        branches: 85,
+        statements: 90,
       },
     },
   },