Remember PKCE fallback clients across VS Code sessions

[clavery]

Summary

• persist a PKCE-unsupported marker after a successful automatic implicit fallback, scoped by client ID and Account Manager host
• reuse the compatible implicit session on later CLI processes and VS Code activations instead of repeating the known-failing PKCE browser flow
• report that authorization was received in the PKCE callback instead of claiming authentication succeeded before token exchange
• document the fallback behavior and migration override

Validation

• pnpm --filter @salesforce/b2c-tooling-sdk run test:agent — 2,259 passing, 7 pending
• pnpm run lint:agent
• pnpm run typecheck:agent
• SDK and VS Code builds
• pnpm run docs:build
• SDK formatting check

Fixes #633

[github-actions]

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