feat(model-catalog): add bedrock_mantle/us-gov-west-1/openai.gpt-5.6-sol

[okxint]

Summary

Closes #40102 (remaining piece).

#40581 added openrouter/openai/gpt-5.6-sol; the issue's last comment noted that bedrock_mantle/us-gov-west-1/openai.gpt-5.6-sol was still missing. This PR adds it.

The entry follows the same structure as the existing us-gov-west-1 siblings (gpt-5.6-terra, gpt-5.6-luna):

• mode: responses, use_openai_responses_path: true
• Same capability flags as bedrock_mantle/openai.gpt-5.6-sol (vision, reasoning, web search, function calling, prompt caching, response schema)
• Pricing derived from standard sol costs using the same 1.2× GovCloud premium applied to the terra sibling

Both model_prices_and_context_window.json and litellm/model_prices_and_context_window_backup.json updated.

[CLAassistant]

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[codspeed]

Merging this PR will not alter performance

✅ 31 untouched benchmarks

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_{Comparing okxint:fix/bedrock-mantle-gov-sol-entry (40b9643) with litellm_internal_staging (9071ca5)}

[mateo-berri]

Thanks for the PR. AWS does not offer GPT-5.6 Sol in GovCloud (its model card and the us-gov-west-1 price list omit it), so closing