Add oidcIssuers getter to Agent

[jeswr]

Summary

Add an oidcIssuers getter on Agent that returns the values of solid:oidcIssuer triples on the WebID profile, mirroring the existing pimStorage / solidStorage / storageUrls getters.

solid:oidcIssuer is the predicate Solid-OIDC mandates for WebID-Issuer Discovery (see Solid-OIDC spec, WebID-Issuer Discovery section). Downstream consumers — @jeswr/solid-reactive-fetch's WebIDProfileAgent and the ODI Solid browser extension's inject.ts — currently subclass Agent purely to add this same getter. Hosting it on the base Agent lets both packages drop the subclass.

The implementation is a one-line addition that delegates to SetFrom.subjectPredicate exactly as solidStorage does, so it picks up the same Set<string> semantics consumers already rely on for storage discovery.

Test plan

• New unit test test/unit/agent.test.ts verifies the getter returns both values when a WebID declares multiple solid:oidcIssuer triples.
• npm test passes (22/22 tests).

[Copilot]

Pull request overview

Adds first-class support for Solid-OIDC WebID Issuer Discovery on the core Agent model by introducing an oidcIssuers getter that reads solid:oidcIssuer values from the WebID profile, consistent with existing storage discovery getters.

Changes:

• Add Agent.oidcIssuers getter backed by SetFrom.subjectPredicate on solid:oidcIssuer.
• Add unit test coverage ensuring multiple solid:oidcIssuer triples are returned as a set.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/webid/Agent.ts	Adds oidcIssuers getter using the same set-based extraction pattern as solidStorage.
test/unit/agent.test.ts	Verifies the getter returns all issuer URLs when multiple issuers are declared.

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[langsamu]

Minor naming consistency request.