Enforce OpenContext HTTP byte limit while streaming

[rissrice2105-agent]

Fixes #135

Summary

• read HTTP response bodies as byte chunks instead of checking UTF-16 string length
• cancel streaming responses immediately when they cross the 5 MiB limit
• keep the configured timeout active until body consumption completes
• add regressions for multibyte over-limit and valid within-limit responses

Validation

• pnpm --dir packages/opencontext test (228 tests passed)
• pnpm exec tsc -p packages/opencontext/tsconfig.json --noEmit