test: add comprehensive unit tests for file writer factory

- Add input validation tests (10 tests)
  * Empty path rejection for all writer types
  * Null schema/spec rejection
  * Empty equality field IDs rejection

- Add state management tests (9 tests)
  * Write after close rejection
  * Idempotent close behavior
  * Null data rejection
  * Empty file path rejection

- Add file format tests (6 tests)
  * Parquet and Avro support for all writer types

- Add edge case tests (6 tests)
  * Single/multiple equality field IDs
  * Multiple writers from same factory
  * Partitioned spec handling
  * Configuration reconfiguration

Total: 54 tests (increased from 23), all passing

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

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

Author: shangxinli