Add test coverage for restricted-relayer context in ETHBridge

[coderabbitai]

Context

This issue tracks adding test coverage for the restricted-relayer context functionality introduced in PR #129.

Task

Add a test case in test/signal/ETHBridge.t.sol that verifies the restricted-relayer logic where:

• context = abi.encode(address(relayer)) instead of empty context
• The specified relayer can relay successfully
• An arbitrary address attempting to relay reverts with InvalidRelayer() error

Dependencies

This should be implemented after PR #119 is merged to avoid potential conflicts.

References

• Original discussion: Make EthBridge unaware of relayer selection logic and allow arbitrary context instead #129 (comment)
• Related PR: Make EthBridge unaware of relayer selection logic and allow arbitrary context instead #129
• Requested by: @nikeshnazareth