Implement StagingValidityConstraints Runtime API method

[slumber]

A corresponding TODO

polkadot/runtime/parachains/src/runtime_api_impl/vstaging.rs

Lines 29 to 31 in 2c215b2

	// TODO [now]: implicit `validity_constraints`. Ensure that `min_relay_parent`
	// never goes lower than the point at which asynchronous backing was enabled.
	// Also, never cross session boundaries.

Called in

polkadot/node/core/prospective-parachains/src/lib.rs

Lines 555 to 569 in 2c215b2

	#[overseer::contextbounds(ProspectiveParachains, prefix = self::overseer)]
	async fn fetch_base_constraints<Context>(
	ctx: &mut Context,
	relay_parent: Hash,
	para_id: ParaId,
	) -> JfyiErrorResult<Option<Constraints>> {
	let (tx, rx) = oneshot::channel();
	ctx.send_message(RuntimeApiMessage::Request(
	relay_parent,
	RuntimeApiRequest::StagingValidityConstraints(para_id, tx),
	))
	.await;
	Ok(rx.await.map_err(JfyiError::RuntimeApiRequestCanceled)??.map(From::from))
	}

[slumber]

cc @rphmeier

[rphmeier]

Yes. Worth noting that in #6014 we leaned heavier on the "Also, never cross session boundaries" part.

[eskimor]

Also make sure to use the new staging API functionality for this.

[slumber]

Resolved by #6258