Implement analyze_single_file_simple helper function

[oleander]

Task

Create a new helper function analyze_single_file_simple in src/multi_step_integration.rs that analyzes a single file without using function calling schemas.

Requirements

• Accept OpenAI client, model string, and file information as parameters
• Return a Result containing the analysis summary
• Construct a straightforward prompt including:
  • File path for context
  • Operation type (addition, modification, deletion, or rename)
  • Actual diff content showing changed lines
• Request plain text output (no function calling or structured output schemas)
• Ask model to summarize in 1-2 sentences what changed and why it matters
• Straightforward error handling (return error if API call fails)
• Log errors at appropriate level

Success Criteria

• Function produces quality summaries in isolation
• Works without function calling overhead
• Clear error messages when API calls fail
• Summaries contain substantive information about changes

Part of #81