fix(mcp): enforce strict wallet network matching in CDP strategy

[Lutra23]

Summary

CDPSigningStrategy currently treats mainnet/testnet as compatible by “family” (e.g. base and base-sepolia) and also falls back to any active wallet when no compatible wallet is found. This blurs network domain separation for payment signing.

This PR enforces strict matching:

• A wallet is only considered compatible when walletNetwork === targetNetwork.
• If there is no matching wallet for the requested network, we return a clear error instead of attempting to sign with a wallet from a different network.

Repro (before)

1. Have only a base-sepolia CDP wallet active.
2. Trigger a payment requirement for base.
3. Strategy considers the wallet compatible (family match) and/or falls back to active wallets, leading to cross-network signing attempts.

Impact

• Prevents accidental cross-network wallet usage (mainnet vs testnet).
• Makes failure modes explicit and actionable (create/activate a wallet on the requested network).

Behavior change (expected)

This is a tightening change: flows that previously “worked by accident” via family/fallback may now return an error if no wallet exists on the target network.

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