Add ACN Mutator (assert(...) Condition Negation)

[sidarth16]

Summary

This PR introduces ACN (Assert Condition Negation) to the mutator set.

The mutator rewrites the first condition argument of assert(...) by wrapping it in a negation, so assertion-style guards can be flipped during mutation testing.

Examples:

• assert(x > 0 && x < 10) becomes assert(!(x > 0 && x < 10))
• assert(msg.sender == owner) becomes assert(!(msg.sender == owner))
• assert(!isOwner) becomes assert(!(!isOwner))

Changes

• Added ACN mutator:
  • slither/tools/mutator/mutators/ACN.py
  • detects assert(...) calls & generates a patch that rewrites the condition as !(...)
• Register ACN mutator in:
  • slither/tools/mutator/mutators/all_mutators.py
• Updated test Contract:
  • Added assert(..) statement in tests/tools/mutator/test_data/test_source_unit/src/Counter.sol
• Added tests in:
  • tests/tools/mutator/test_mutator.py testing the ACN mutator

Notes

• This PR is intentionally limited to assert(...) mutations.
• The require(...) variant is being handled separately in PR Add RCN Mutator (require(...) Condition Negation) #2998.
• ACN is listed with # severity medium in all_mutators.py for consistency with the existing mutator list comments.
• ACN is counted as tweak mutant by current framework rules (RR/CR special-cased)

Fixes #2999