Implement Featherless chat completions adapter

[Photon101]

Summary

• replace the broken Featherless stub with a real OpenAI-compatible chat completions adapter
• use the documented https://api.featherless.ai/v1/chat/completions endpoint, documented example model IDs, dry-run behavior, standard generation options, and opts.extra passthrough for sampler parameters
• send Featherless-requested HTTP-Referer and X-Title application headers
• add focused tests for dry-run behavior, request payload/auth/attribution headers, token usage mapping, and HTTP error excerpts

References

• https://featherless.ai/docs/completions
• https://featherless.ai/docs/quickstart-guide
• https://featherless.ai/docs/api-overview-and-common-options

Validation

• corepack pnpm exec vitest run packages/ai/featherless/src/index.test.ts
• corepack pnpm exec tsc -p packages/ai/featherless/tsconfig.json --noEmit
• git diff --check