Add slas token command for SLAS shopper access tokens

[clavery]

Summary

• Adds b2c slas token command to retrieve SLAS shopper access tokens for API testing
• Supports public client (PKCE), private client (client_credentials), guest and registered customer flows
• Auto-discovers the first public SLAS client when --slas-client-id is omitted
• Adds slasClientId, slasClientSecret, and siteId to the config system (dw.json, env vars, flags)
• New SDK module (@salesforce/b2c-tooling-sdk/slas) with typed token retrieval functions and PKCE helpers
• JSON output separates response (raw SLAS fields) from decodedJWT (decoded token payload)
• Fixes test isolation: stubParse now defaults config: '/dev/null' to prevent dw.json leakage
• Adds parseRedirectUris() helper to handle pipe-delimited redirect URIs from the SLAS API

Test plan

• SDK PKCE tests (verifier length, base64url encoding, challenge correctness)
• SDK token flow tests (public guest PKCE, private guest client_credentials, registered public, registered private, error handling) via MSW
• CLI command tests (JSON output structure, client_credentials, auto-discovery error, missing flags, registered flow, decodedJWT presence/absence)
• All 972 CLI tests pass
• All SDK tests pass
• Lint clean (pnpm run lint:agent)

[clavery]

@amit-kumar8-sf any reason why you merged main into my PR?