Add Haiku 5.5 as a provider model for direct Anthropic access #2193

Name of model: Haiku 5.5
Model creator: Anthropic
Link to model: https://platform.claude.com/docs/en/models/haiku-5-5/migration-guide#use-the-claude-haiku-5-5-model-id

This is analogous to #2100, but for people using Anthropic API keys rather than AWS Bedrock API keys. The Haiku 5.5 model is not available in the model selection dropdown for Anthropic.

Currently users can specify the Haiku 5.5 model string directly, but doing so doesn't enable prompt caching, making it about 10x as expensive.

Haiku 5.5 can be used via AWS Bedrock, but (a) that only applies to users using Bedrock, and (b) using 5.5 on Bedrock requires special approval (the default rate limit is set to 0), whereas using the model directly from Anthropic does not require AWS approval.