add additional instance methods

[DonPrus]

Summary

Adds additional instance-scoped methods for gateway-backed components.

What Changed

• Adds generic instance proxy routes for agent-stream, a2a, a2a-stream, and transcribe.
• Ensures NullHub prepares and starts the selected instance before gateway proxying.
• Stores the NullHub-managed bearer token as a sidecar secret and writes only its SHA-256 hash into gateway pairing config.
• Adds true SSE passthrough for streaming A2A traffic, including upstream chunked transfer handling.
• Adds wizard support for generic stateless runtime profile hints: memory disabled, A2A multimodal enabled, media-sized gateway limits, and gateway pairing provisioned.
• Raises the HTTP body limit for media payloads while keeping the initial request stack buffer small.

Validation

• zig version -> 0.16.0
• zig build test --summary all
• zig build --summary all
• zig build test-integration --summary all
• zig build -Dtarget=x86_64-windows-gnu -Dembed-ui=false --summary all
• zig build -Dtarget=x86_64-linux-gnu -Dembed-ui=false --summary all
• git diff --check