mcphttp: the MCP endpoint plumbing both services already share

hostAllowed is byte-identical in cov and bench, and neither should own it.
PrivateCache is cov's alone today, which is why bench's /mcp currently sets
no Cache-Control and no Vary at all. StreamableOptions carries the pair both
pass and the reason for it.

HostGuard takes cov's fail-closed stance: an origin with no host to guard
with is a construction error, not a warning. bench serves unguarded in that
case and has a test pinning it; reconciling that is bench's to do.

Deliberately not a shared MCP framework. Once the go-sdk and the per-service
tools are subtracted, this is the whole overlap.