feat(providers): add progress-based upstream request timeout

Adapters previously issued upstream requests with no timeout at all.
A provider that accepted a connection and then stalled hung the
gateway request forever. newAdapterHTTPClient now sets
Transport.ResponseHeaderTimeout, and every response body read
(providers.go's upstreamBytes, routes_passthrough.go's proxyUpstream)
runs through a watchdogBody that resets on every successful read and
cancels the request once timeout passes with no progress.

The idle-progress design lets a long stream that keeps emitting tokens
run indefinitely. Only silence, never total duration, ends a request.

Config.RequestTimeout sets the global default (five minutes when
unset). ProviderConfig.RequestTimeout overrides it per provider. Both
are Go duration strings, validated at construction; a zero or negative
value is rejected rather than treated as unlimited, since that would
reintroduce the hang this feature fixes.

Native passthrough (routes_passthrough.go) and the MCP/A2A target
proxy (mcp_a2a.go) share the identical watchdog mechanism through
proxyUpstream's new timeout parameter.

A watchdog-fired timeout wraps errProviderTimeout and names the
provider. It never wraps context.Canceled, so it stays distinguishable
from a genuine client disconnect in every existing log classification.

mcp_a2a.go

@@ -216,7 +216,7 @@ func (g *Gateway) handleTargetProxy(w http.ResponseWriter, r *http.Request, u *u
 		upstreamURL += "?" + r.URL.RawQuery
 	}
 
-	g.proxyUpstream(w, r, upstreamURL, g.targetClient, nil, nil, false, kind+" target (name "+name+")")
+	g.proxyUpstream(w, r, upstreamURL, g.targetClient, nil, nil, false, kind+" target (name "+name+")", g.targetTimeout)
 }
 
 // validateTargetURLs checks every configured MCP-server and agent entry at