HttpBridge should auto-register Arrow decoder when available

[bbopen]

Problem

HttpBridge decodes responses via decodeValueAsync, but never registers an Arrow decoder. When the server returns Arrow-encoded envelopes, callers see Arrow decoder not registered errors even in Node environments where apache-arrow is available. NodeBridge auto-registers; HttpBridge does not.

Evidence

• src/runtime/http.ts: no call to autoRegisterArrowDecoder or registerArrowDecoder.

Acceptance criteria

• Attempt autoRegisterArrowDecoder during HttpBridge construction or before first call (Node/Bun only).
• Keep behavior safe for browser builds (no hard dependency on apache-arrow).
• Add tests for Node runtime where Arrow is available and for browser builds where it is not.