[FEATURE] Multi-account session management

Problem

Consultants and developers who work across multiple Microsoft Fabric tenants and client environments face a painful workflow today: every time they need to switch context, they must run fab auth logout followed by a full fab auth login, re-authenticating interactively each time.

Real-world use case

A typical consulting scenario:

• Client A - Production tenant tenant-aaa, user consultant@clientA.com
• Client B - Dev/test tenant tenant-bbb, user consultant@clientB.com
• Internal - Corporate tenant tenant-ccc, user me@mycorp.com

Switching between these today requires a full logout/login cycle for each transition, which is disruptive, slow, and breaks automation scripts that assume a persistent session.

Proposed solution

Add stored user session management to the CLI so that multiple interactive authentication sessions can coexist and be switched instantly:

Behavior

• Two stored sessions, no flags: fab auth switch automatically toggles to the other account.
• Three or more sessions, no flags: an interactive prompt is shown.
• With -u (and optionally -t): switches directly, no prompt.
• Sessions are validated against the MSAL token cache and marked as valid/invalid in fab auth list.
• Environment variable authentication (FAB_TOKEN, etc.) takes precedence and blocks session switching with a clear error message.
• Interactive login now preserves the broker-backed MSAL app across the Fabric, OneLake, and Azure token warm-up so newly selected accounts are stored cleanly on Windows.

Acceptance criteria

• fab auth list shows all stored sessions with active/valid/last_used metadata
• fab auth switch supports direct (-u, -t) and interactive selection
• fab auth logout supports per-session removal (-u, -t) and bulk (--all)
• Username matching is case-insensitive
• Session switching is blocked when env var auth is active
• Documentation and examples are up to date
• Tests cover happy paths and edge cases