feat: add access token methods

[gao-sun]

Summary

• add access token helpers to LogtoClient (resource + organization, plus “claims” variants)
  • getAccessToken(for:organizationId:): Returns an access token for resource (optional) and organizationId (optional). Uses cached token if valid; otherwise refreshes.
  • getAccessTokenClaims(for:organizationId:): Returns decoded access token claims for resource and organizationId.
  • getOrganizationToken(forId:): Returns an organization-scoped access token for id.
  • getOrganizationTokenClaims(forId:): Returns decoded organization token claims for id.
• implement underlying fetch / utilities for token exchange and parsing
• expand SwiftUI demo with placeholder-based config + buttons to fetch access/organization token claims

Testing

test cases added/updated

[Copilot]

Pull request overview

This PR extends the SDK’s access token handling by introducing helpers for decoding access token claims, supporting organization-aware token fetching, and adding supporting tests and utilities.

Changes:

• Add LogtoUtilities.decodeAccessToken and JsonValue convenience accessors to work with structured access token claims.
• Extend LogtoClient access token APIs to support organization-specific tokens and new helpers to fetch access token/organization token claims, including tests and mock responses.
• Refactor LogtoCore.fetchToken (refresh-token flow) to centralize payload construction via buildFetchTokenPayload, and add tests to validate the new behavior.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
Tests/LogtoTests/LogtoUtilitiesTests.swift	Adds tests for decoding access tokens (including nested claims) and error handling for malformed tokens.
Tests/LogtoTests/LogtoCoreTests+Fetch.swift	Updates refresh-token fetch tests to include the new organizationId parameter.
Tests/LogtoMock/NetworkSessionMock.swift	Extends the mock session with a reusable JWT access token and new OIDC/token endpoints for JWT-based flows.
Tests/LogtoClientTests/LogtoClient/LogtoClientTests+Fetch.swift	Adjusts access token cache key usage and adds tests for LogtoCore.buildFetchTokenPayload with resource/organization combinations.
Tests/LogtoClientTests/LogtoClient/LogtoClientTests+AccessToken.swift	Updates cache key construction to include organization ID and adds tests for organization token fetching and access token claims.
Sources/LogtoClient/LogtoClient/LogtoClient+SignIn.swift	Ensures the initial access token after sign-in uses the new (resource, organizationId)-based cache key.
Sources/LogtoClient/LogtoClient/LogtoClient+AccessToken.swift	Introduces organization-aware access token keying, extends getAccessToken to accept organizationId, and adds claim-access helpers for resources and organizations.
Sources/Logto/Utilities/LogtoUtilities+AccessToken.swift	Adds a utility to decode access token JWT payloads into JsonObject without validation.
Sources/Logto/Types/Json.swift	Adds convenience accessors (stringValue, numberValue, etc.) and isNull to JsonValue for easier claim inspection.
Sources/Logto/Core/LogtoCore+Fetch.swift	Refactors refresh-token token fetching to use a shared buildFetchTokenPayload that handles organization URNs and explicit organization IDs.

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