Error Message Improvements

What: More specific error types and clearer user feedback

New error types:

• repeated_529 - Rate limit errors with specific guidance
• prompt_too_long - Oversized prompts with size information
• pdf_too_large - PDF page limit violations
• oauth_org_not_allowed - OAuth restriction details

Details:

• Better categorization of error conditions
• More actionable error messages
• Context-specific troubleshooting hints
• Evidence: New error types at lines 1499-1554 in diff