fix: add CJS declarations for HTTP and LangChain exports

[sjungwon03]

Summary

• Fixes Node16/CommonJS TypeScript static imports of @theorvane/type-mcp/http and @theorvane/type-mcp/langchain, which previously selected ESM declarations and failed with TS1479 despite working CJS runtime exports.
• Adds nested import/require export conditions for root, HTTP, and LangChain entrypoints; CommonJS now selects generated .d.cts declarations and .cjs runtime files.
• Releases this package-contract correction as @theorvane/type-mcp@0.3.1.
• Extends packed Node16 CommonJS regression coverage to compile and execute legacy decorators together with static HTTP/LangChain imports, and verifies CJS declaration artifacts in package and publish checks.
• Documents the Node16 CJS import contract and the required @langchain/core peer for the LangChain adapter.

Verification

• npm run typecheck
• npm run lint
• npm test — 61 tests
• npm run verify:package
• npm run verify:consumer — packed Node16 CommonJS compile/runtime regression
• npm run verify:publish
• npm audit --omit=dev --audit-level=low — 0 vulnerabilities
• git diff --check

Closes #138

[sjungwon03-ai]

Verified the exact Node16 CJS declaration head: http/langchain/root conditionally select generated .d.cts declarations for require, packed CJS static imports compile and execute with the LangChain peer, and package/publish artifact checks cover both declaration forms.