fix(azure): four clients report "no existing commitments" when the pager cannot be built

[cristim]

Summary

Four Azure clients log a pager-construction failure and return an empty commitment slice with a nil error. A caller comparing recommendations against existing commitments concludes the subscription holds no reservations and recommends or executes a duplicate purchase. Two sibling clients return the error on the same path, so the behaviour is inconsistent across the provider. The verifier narrowed the blast radius: the discarded error comes only from client construction, so a permission failure surfaces later at the first page fetch rather than here.

Location

providers/azure/services/cache/client.go:185 (same at cosmosdb/client.go:187, database/client.go:217, search/client.go:132) at 3c0f8ac94048a2c36fce5ccddee54e6c4849a5cd

Failure scenario

A credential, permission or client-construction failure is logged to stdout and converted into an empty slice with a nil error. A caller comparing recommendations against existing commitments concludes the subscription holds no reservations and recommends (or executes) a purchase that duplicates an existing one. synapse/client.go:168 and managedredis/client.go:179 return the error on this same path, so the behaviour is inconsistent across the provider.

Evidence

pager, err := c.createReservationsPager()
if err != nil {
	log.Printf("WARNING: failed to create Redis reservations pager: %v", err)
	return []common.Commitment{}, nil
}

Suggested fix

Return the wrapped error, matching synapse and managedredis.

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Found by the 2026-09-02 codebase audit, finding A08b-022, reported by one reviewer and independently confirmed by a second. Full report: docs/audits/codebase-audit-2026-09-02.md.