bedrock: Add support for GPT 5.6 models

[IbrahimKhan12]

Body

What are you proposing?

GPT 5.6 models are now available on AWS Bedrock:

https://docs.aws.amazon.com/bedrock/latest/userguide/model-card-openai-gpt-56-sol.html
https://docs.aws.amazon.com/bedrock/latest/userguide/model-card-openai-gpt-56-terra.html
https://docs.aws.amazon.com/bedrock/latest/userguide/model-card-openai-gpt-56-luna.html

Why does this matter?

Zed should support all AWS Bedrock models, especially the newest ones.

What problem does this solve?
Zed users who access OpenAI models through AWS Bedrock currently cannot select GPT-5.6 Sol, Terra, or Luna as built-in models.

What becomes easier or possible?
Access to GPT 5.6, a frontier model.

Are there any examples or context?

Native Bedrock Mantle support has already been added to Zed: #60480

Possible approach

Add them as three variants to MantleModel in: crates/bedrock/src/models.rs

The relevant MantleModel methods and test cases should be updated to support these.

[IbrahimKhan12]

I have a PR ready for this: #61008