Improve error reporting for when requests are dropped because of saturation.

[ahg-g]

For sheddable requests, we currently return a blank error code and message that the request was dropped due to "system saturated": https://github.com/kubernetes-sigs/gateway-api-inference-extension/blob/main/pkg/epp/requestcontrol/director.go#L170

This doesn't distinguish between the case where we don't have any endpoints to select from (no ready pods) and the case where we do, but are saturated.

/assign @LukeAVanDrie

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In response to this:

For sheddable requests, we currently return a blank error code and message that the request was dropped due to "system saturated": https://github.com/kubernetes-sigs/gateway-api-inference-extension/blob/main/pkg/epp/requestcontrol/director.go#L170

This doesn't distinguish between the case where we don't have any endpoints to select from (no ready pods) and the case where we do, but are saturated.

/assign @LukeAVanDrie

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