fix: remove inconsistent haiku_4_5 underscore variant from structured output allowlist

The underscore variant has no evidence of real usage in the codebase or
Anthropic's model naming conventions. All existing model name formats
(standard, Bedrock, provider-prefixed) contain the dash form 'haiku-4-5'
as a substring, which is already covered. The underscore entry breaks
the pattern set by all other model entries (sonnet, opus) which only
use dash and dot forms.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

litellm/llms/anthropic/chat/transformation.py

@@ -991,7 +991,6 @@ class AnthropicConfig(AnthropicModelInfo, BaseConfig):
                     for substring in {
                         "haiku-4-5",
                         "haiku-4.5",
-                        "haiku_4_5",
                         "sonnet-4.5",
                         "sonnet-4-5",
                         "opus-4.1",