feat(contract): implement multi-wallet session state switching

[mijinummi]

Description

This PR addresses Issue #374, delivering a secure multi-wallet session switching mechanism inside the Soroban smart contract framework.

Key Modifications

• Atomic State Purging: Built state-management keys using env.storage().persistent() to drop previous address identifiers when a hot-swap occurs.
• Mid-Flow Transaction Guard: Added an IsFlowLocked verification wrapper that blocks wallet context modification if an operation is midway through an atomic execution block.
• On-Chain Network Verification: Enforced validation gates that parse incoming network symbols to shield state machines from cross-network injection attacks.

Closes #374

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[ayshadogo]

LGTM