Allow explicit self return in no-op methods

[Gabriel-Darbord]

Summary

• only report MCPRedundantExplicitSelfReturnRule when a final ^ self follows another top-level statement
• accept a method whose complete body is ^ self as an intentional no-op
• update the rule rationale and focused method-compile coverage

Verification

• focused explicit-self-return tests: 2 passed
• Pharo 13 MCP-Tests and MCP-UI-Tests: 718 passed
• git diff --check passes