chore: clean up invariant test files

[jackchuma]

Summary

Cleans up the invariant tests for CrossDomainMessenger and SafeCall to reduce duplication and clarify intent.

• Refactor CrossDomainMessenger.t.sol: collapse the success/failure invariants down to a single badRelayResult flag, move shared setup into a base contract, drop unused imports/sender exclusions, and tighten naming/comments.
• Refactor SafeCall.t.sol: introduce a shared SafeCall_Invariants base, replace the numCalls counters with a badCallResult flag, extract magic numbers into named constants, and use assumeUnusedAddress instead of hand-rolled to.code.length checks.

No behavioral changes to the invariants themselves — same scenarios are exercised, just with less boilerplate.

[cb-heimdall]

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