feat: add Cheaper Inference provider preset

[aiapienthusiast]

This PR adds Cheaper Inference as an API provider preset.
It uses the same pattern as the Atlas Cloud part of #170.

Cheaper Inference is an OpenAI-compatible LLM gateway.
One API key gives access to models from several labs.

Usage: open Settings, then Providers.
Add "Cheaper Inference" and paste an API key from https://cheaperinference.com/signup.
Models load from GET /v1/models (for example gpt-5.4-mini).

Details

Files:

• src/shared/providerCatalog.ts: new cheaperinference preset (https://api.cheaperinference.com/v1, key required). It is also in POPULAR_KINDS, after astraflow.
• webview-ui/sidebar/types.ts: cheaperinference in ModelKind.
• src/agent/provider.test.ts: cheaperinference in the expected presets.

Tests:

• pnpm run check-types: pass.
• eslint on the changed files: pass.
• vitest run for provider.test.ts, providerCatalog.test.ts, providerRouting.integration.test.ts, apiKeyPool.test.ts, sidebarProvider.test.ts: 136 passed.
• node scripts/check-secrets.mjs: pass.

Live check: gpt-5.4-mini returned a reply through streamChat with the preset base URL. listModels returned the catalog.