feat: Add error logging and fix incremental migration state tracking

Implement comprehensive error handling and fix state management bug in incremental migration:

Error Logging System:
- Add validation for consolidation keys (NULL dates, empty IDs, corrupted Java strings)
- Log invalid keys to dedicated error files with detailed reasons
- Full migration: migration_errors_<table>_<partition>.log
- Incremental migration: migration_errors_<table>_incremental_<timestamp>.log (timestamped to preserve history)
- Report total count of skipped invalid keys at migration completion
- Auto-delete empty error log files

State Tracking Fix:
- Fix critical bug where last_key wasn't updated after final buffer flush
- Track last_processed_key throughout migration loop
- Update state both during periodic flushes and after final flush
- Ensures incremental migration correctly resumes from last migrated key

Validation Checks:
- EventDate IS NULL or EventDate = '0000-00-00'
- EventTime IS NULL
- ToolNameID IS NULL or empty string
- UnitName IS NULL or empty string
- UnitName starting with '[L' (corrupted Java strings)

Documentation:
- Update README.md with error logging behavior
- Update MIGRATION_WORKFLOW.md with validation details
- Update CHANGELOG.md with new features and fixes

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

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

src/utils/validation.py

@@ -10,21 +10,30 @@ logger = get_logger(__name__)
 class ErrorLogger:
     """Log invalid migration keys to a file."""
 
-    def __init__(self, table: str, partition: str):
+    def __init__(self, table: str, partition: str, use_timestamp: bool = False):
         """Initialize error logger.
 
         Args:
             table: Table name
-            partition: Partition name
+            partition: Partition name (e.g., 'p2024' or 'incremental')
+            use_timestamp: If True, add timestamp to filename (for incremental migrations)
         """
         self.table = table
         self.partition = partition
-        self.error_file = f"migration_errors_{table}_{partition}.log"
+
+        # Add timestamp to filename for incremental migrations to avoid overwriting
+        if use_timestamp or partition == "incremental":
+            timestamp = datetime.now().strftime("%Y%m%d_%H%M%S")
+            self.error_file = f"migration_errors_{table}_{partition}_{timestamp}.log"
+        else:
+            self.error_file = f"migration_errors_{table}_{partition}.log"
+
         self.error_count = 0
 
         # Create error file with header
         with open(self.error_file, "w") as f:
             f.write(f"# Migration errors for {table} partition {partition}\n")
+            f.write(f"# Timestamp: {datetime.now().strftime('%Y-%m-%d %H:%M:%S')}\n")
             f.write("# Format: UnitName|ToolNameID|EventDate|EventTime|Reason\n\n")
 
         logger.info(f"Error log file created: {self.error_file}")
@@ -99,6 +108,10 @@ def validate_consolidation_key(
     if unit_name is None or unit_name == "":
         return False, "UnitName is NULL or empty"
 
+    # Check for corrupted Java strings (like '[Ljava.lang.String;@...')
+    if isinstance(unit_name, str) and unit_name.startswith("[L"):
+        return False, f"UnitName is corrupted Java string: {unit_name}"
+
     if tool_name_id is None or tool_name_id == "":
         return False, "ToolNameID is NULL or empty"