refactor: chat_generation cleanup and bug fixes

[marcelo-earth]

Summary

• Bug fix: manage_conversation_images returned len(int) for non-openai engines (TypeError waiting to happen) — corrected to return the count directly
• Bug fix: removed unused BlobServiceClient import (Azure import with no call site in this file)
• Refactor: hoisted ENGINE_DEFAULTS and VALID_MODELS from inside the route handler to module-level constants
• Refactor: removed all 45 debug print statements from the route (request dumps, message history logs, chunk-by-chunk traces, tool call traces, base64 debug dumps)
• Refactor: removed commented-out dead code from the OpenAI streaming branch (two yield blocks and a pass-only if/else)
• Refactor: removed obvious what-comments and multi-line function docstring from route handler
• Test: removed duplicate app/client fixture definitions from test_litellm_provider.py (now covered by conftest.py)

Net: -206 lines, 0 behavior changes.

Test plan

• Full suite: 141 passed, 1 warning before and after every commit