feat: support if err = doSomething(); err != nil

[fooree]

• Description

  Extend the noinlineerr analyzer to detect and autofix if err = ...; err != nil patterns (reassignment), not just := declarations.

• Motivation

  The analyzer previously only flagged if err := f(); err != nil { but missed the equivalent reassignment form if err = f(); err != nil {, which violates the same readability principle — inline
  error handling should be split into a standalone assignment followed by a separate if check. This closes the gap for reassignments that reuse an already-declared err variable.
  I

• Implementation Details

  • Changed errMessage from a const to a function parameterized by token.Token so the diagnostic message uses := or = depending on the assignment token.
  • Fixed the shadow-check guard condition at :71 — the original if len(Lhs) != 1 || shadowVarsExists(...) matched the = case (which has no shadow concern) but the token.DEFINE condition wasn't
    checked, causing the = case to fall through correctly. Added assignStmt.Tok == token.DEFINE && to the second branch so it only gates := with shadow.
  • Updated all errMessage call sites to pass assignStmt.Tok.
  • Added test function inlineErrReassign in testdata with a // want comment and corresponding golden file entry verifying the autofix output.

• Test Coverage

  • Added inlineErrReassign test case covering if err = ...; err != nil pattern.
  • Golden file updated to verify the autofix transforms it to err = ... + standalone if.