Implement Inception chat completions adapter

[Photon101]

Summary

• Replace the Inception BYOK stub with a real Mercury 2 chat completions adapter
• Use INCEPTION_API_KEY, the documented Inception v1 chat endpoint, dry-run short-circuiting, usage token mapping, and response body excerpts on HTTP errors
• Add focused tests for dry-run, request payloads, base URL override, token mapping, and errors

Validation

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