fix: Address PR #186 code quality issues

Issue #5: System property restoration in TestApplication
- Added previousSkipEffectivePomValue field to store previous property value
- Modified useEffectivePomGeneration() to save previous value before clearing
- Added cleanup() method to restore system property after tests
- Prevents test pollution where one test's effective POM setting affects others

Issue #6: Silent fallback on Maven failure in MuleApplication
- Changed getEffectivePomFile() to throw RuntimeException instead of silently falling back
- Provides clear error message with Maven exit code and troubleshooting hint
- Fail-fast approach prevents rules from silently operating on incomplete POM data

Issue #8: Temp file leak on Maven exception
- Moved deleteOnExit() registration immediately after temp file creation
- Wrapped Maven invocation in try-catch to ensure temp file cleanup on exception
- Prevents effective-pom temp files from accumulating in /tmp on Maven failures

All changes verified with successful build.

Author: adesjardin