Add redundant-assert checker for implied assert.Error assertions

[mmorel-35]

This adds a new checker to catch redundant assert.Error / assert.Errorf calls when a stronger error assertion on the same error expression already exists in the same block. The goal is to remove duplicate signal and keep error assertions minimal.

• New checker: redundant-assert

  • Introduces an advanced checker that scans assertion calls per ast.BlockStmt.
  • Reports redundant assert.Error* when paired with a stronger assertion on the same error expression:
    • ErrorContains*
    • ErrorIs*
    • ErrorAs*
    • EqualError*
  • Uses expression-string matching (analysisutil.NodeString) for same-error detection.
  • Suggests an autofix that removes the redundant statement.

• Checker integration

  • Registers redundant-assert in checker priority order and enables it by default.
  • Updates registry/factory expectations in checker/analyzer tests.

• Docs and examples

  • Adds redundant-assert to the README checker table.
  • Adds a dedicated README section with invalid/valid patterns and autofix/default metadata.

• Analyzer test coverage

  • Adds checker-specific testdata (.go + .golden) for:
    • assert.Error* + stronger assertion on same error (diagnostic expected)
    • assert object style (assObj.Error)
    • assert/require mixed stronger assertions
    • non-matching cases (different vars / different blocks / standalone checks)

// before
assert.Error(t, err)
assert.ErrorContains(t, err, "not found")

// after
assert.ErrorContains(t, err, "not found")