docs(binding-coap): document PSK usage with CoAPs

[Pranav-0440]

This PR documents PSK usage with CoAPs in the binding-coap README.

Currently, PSK support is implemented in CoapsClient but not documented.

This change adds:

• Thing Description example with psk scheme
• Client configuration example
• Explanation of required identity and psk fields

Closes #954

[Copilot]

Pull request overview

Adds documentation to the CoAP binding README describing how to use DTLS PSK with coaps://, addressing the gap noted in issue #954.

Changes:

• Document PSK support for CoAPs (coaps://) and where it is implemented.
• Add a TD snippet showing a psk security scheme.
• Add a client-side configuration example for PSK credentials.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

The example uses CoapClientFactory, but coaps:// URIs require registering CoapsClientFactory (scheme is coaps). With only CoapClientFactory, WoT.requestThingDescription("coaps://...") / interactions will fail because the Servient has no client factory for the coaps scheme. Update the import and addClientFactory call to use CoapsClientFactory for this section (or register both factories if you want to support both schemes in the same script).

Suggested change

	const { CoapClientFactory } = require("@node-wot/binding-coap");
	const servient = new Servient();
	servient.addClientFactory(new CoapClientFactory());
	const { CoapsClientFactory } = require("@node-wot/binding-coap");
	const servient = new Servient();
	servient.addClientFactory(new CoapsClientFactory());

[Copilot]

thing.setSecurity(...) is not part of the node-wot ConsumedThing API, and td.securityDefinitions is not the type expected by protocol-client setSecurity (it expects an array of resolved SecuritySchemes). In node-wot, credentials should be provided via servient.addCredentials(...) keyed by the Thing id (td.id), and the runtime will apply them automatically based on the TD's security/securityDefinitions when the client is selected.

Suggested change

	const thing = await WoT.consume(td);
	// Configure PSK credentials
	thing.setSecurity(td.securityDefinitions, {
	identity: "Client_identity",
	psk: "secretPSK",
	});
	// Configure PSK credentials for this Thing
	servient.addCredentials({
	[td.id]: {
	identity: "Client_identity",
	psk: "secretPSK",
	},
	});
	const thing = await WoT.consume(td);