[FEATURE]: Add CheaperInference to the default provider list

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Describe the enhancement you want to request

CheaperInference aggregates and routes LLM requests across marketplace nodes to offer lower costs under direct API list price ceilings. It uses standard OpenAI Chat Completions and Anthropic Messages formats, so Kilo Code already functions with it through the Custom Provider settings.

• Base URL: https://cheaperinference.com
• API Key: <cheaperinference key>
• Documentation: https://www.cheaperinference.com/docs

Why native listing is needed: Similar to #14100 and #13471, because CheaperInference features fluid, dynamic marketplace pricing, users cannot track token costs accurately using static local configurations like kilo.jsonc.

To support accurate UI spending metrics, CheaperInference returns real-time settlement data in successful response payloads. Kilo can natively map costs via:

• The response body JSON: usage.cost or cheaper_inference.billing.billed_cost_usd
• The response headers: x-ci-saved-usd

Opening this in case you want it named in the default provider list. Happy to submit a PR for the provider configuration if you're open to it!