Build the runtime-neutral HTTP fetch handler for B2 MCP.
Optional credential, server factory, capability, and MCP handler overrides for tests or serverless adapters.
Runtime-neutral B2 MCP fetch handler.
The handler owns credential resolution, host/origin checks, body caps, rate limiting, in-flight caps, capability discovery, MCP handler dispatch, cache sweeps, and graceful drain state.
Build the runtime-neutral HTTP fetch handler for B2 MCP.