fix: avoid_unnecessary_return_variable

[solid-illiaaihistov]

Closed #236

[gemini-code-assist]

Code Review

This pull request updates the avoid_unnecessary_return_variable lint rule to traverse the enclosing FunctionBody instead of just the immediate parent block of the return statement. This correctly handles cases where a variable is declared in an outer block and returned within a nested block (such as an if statement), preventing false positives. A test case has been added to verify this behavior. No review comments were provided, and the changes look correct.

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[danylo-safonov-solid]

what about cases like:

final a = 3;
if (b) {
  return a;
}

a is still unnecessary here. Let's also add a test case like that