Add RCN Mutator (require(...) Condition Negation)

[sidarth16]

Summary

This PR adds a new mutator: RCN (Require Condition Negation).

RCN negates the first condition argument of require(...) calls.

Examples:

• require(x > 0 && x < 10) -> require(!(x > 0 && x < 10))
• require(msg.sender == owner, "not owner") -> require(!(msg.sender == owner), "not owner")
• require(!isOwner, "owner-not-allowed") -> require(!(!isOwner), "owner-not-allowed")

Changes

• Added RCN mutator:
  • slither/tools/mutator/mutators/RCN.py
• Registered RCN in:
  • slither/tools/mutator/mutators/all_mutators.py
• Added tests in:
  • tests/tools/mutator/test_mutator.py
    • mutator discovery check (_get_mutators(["RCN"]))
    • patch generation check for require(...) condition negation

Notes

• This is scoped to require(...) only.
• assert(...) condition negation (ACN) will be handled in a separate PR.
• RCN is listed with # severity medium in all_mutators.py for consistency with the existing mutator list comments.
• RCN is counted as tweak mutant by current framework rules (RR/CR special-cased)

Fixes #2997