Update forking-mainnets.mdx

[BahadorGh]

Making the required corrections for having an applicable cross-chain message passing with ccip in foundry.

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Closing issues

closes #2804

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Description

In the current docs of testing cross-chain message passing in the Local Simulator to fork mainnets of ccip foundry section, there is some issues related to the complete implementation of the ExampleTest.

Issues:

• As the feeToken: address(0), so we need to send the fee amount in the ccipSend
• The polygon chain selector id is not correct to be used for setting the NetworkConfig. it needs to be the real chain id
• The ccipReceiverAddress needs to be set correctly, which is not implemented to verify if the cross-chain from source to destination chain is working as expected.

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Changes

• Checking the fee required for the message passing
• Correctly pay the required feeToken
• Implementing the mock recipient address in the Polygon(destination) chain
• Verifying correct cross-chain message passing
• Correctly set the required chain link ids to setNetworkDetails

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