feat: add runbooks.Get for space-wide runbook queries (cherry-pick of #433)

[NickJosevski]

Cherry-pick of #433 by @Scott-Emberson, re-hosted on a branch in this repo solely so CI can run. Authorship is preserved on the commit — this is a mechanical move, not a rewrite.

Fork PRs cannot satisfy the required test check: integration-tests.yml needs secrets.DB_IMAGE_SA_PASSWORD, OD_IMAGE_ADMIN_API_KEY and OCTOPUS_SERVER_BASE64_LICENSE, and GitHub withholds secrets from pull_request runs originating on a fork, so the job dies at Initialize containers before any Go executes. #433 is red for that reason and no other.

What it does

Six lines of production code plus a test file. The space-wide runbooks URI template and the RunbooksQuery filter struct both already exist in the package — nothing connected them, so RunbooksQuery is currently unreferenced. This adds Get, using the same newclient.GetByQuery one-liner that tenants.Get and the other resources on this template already use:

func Get(client newclient.Client, spaceID string, runbooksQuery RunbooksQuery) (*resources.Resources[*Runbook], error) {
	return newclient.GetByQuery[Runbook](client, template, spaceID, runbooksQuery)
}

Purely additive — no existing signature changes. Callers could previously only reach runbooks through the project-scoped List.

Unblocks a runbooks data source in the Terraform provider (OctopusDeploy/terraform-provider-octopusdeploy#209).

Once this merges

Close #433 pointing here, so Scott gets the credit and knows it wasn't rejected.