Make SAX parsing consistent on all adapters

[HassanAkbar]

This PR is for addressing lutaml/lutaml-model#641

Status matrix

main:

Method	nokogiri	ox	oga	rexml	libxml
SAX parse — attr	❌	✅	❌	❌	❌
SAX parse — text	✅	✅	❌	✅	✅
DOM parse — attr	✅	✅	❌	✅	✅
DOM parse — text	✅	✅	❌	✅	✅
Serialize	✅	✅	❌	✅	✅

current:

Method	nokogiri	ox	oga	rexml	libxml
SAX parse — attr	✅	✅	✅	✅	✅
SAX parse — text	✅	✅	✅	✅	✅
DOM parse — attr	✅	✅	✅	✅	✅
DOM parse — text	✅	✅	✅	✅	✅
Serialize	✅	✅	✅	✅	✅

Actual outputs (per adapter)

nokogiri

  SAX parse - attr
- main:   Ampersand: & Hex: & Copyright: ©
+ branch: Ampersand: & Hex: & Copyright: ©

  SAX parse - text     (unchanged)  Ampersand: & Hex: & Copyright: ©
  DOM parse - attr     (unchanged)  Ampersand: & Hex: & Copyright: ©
  DOM parse - text     (unchanged)  Ampersand: & Hex: & Copyright: ©
  Serialize            (unchanged)  <doc details="Ampersand: &amp;#38; Hex: &amp;#x26; Copyright: &amp;#169;"></doc>

ox

  All paths already correct on main; branch is identical.
  Parse:     Ampersand: & Hex: & Copyright: ©
  Serialize: <doc details="Ampersand: &amp;#38; Hex: &amp;#x26; Copyright: &amp;#169;"></doc>

oga

  SAX parse - attr
- main:   Ampersand: & Hex: & Copyright: ©
+ branch: Ampersand: & Hex: & Copyright: ©

  SAX parse - text
- main:   Ampersand: & Hex: & Copyright: ©
+ branch: Ampersand: & Hex: & Copyright: ©

  DOM parse - attr
- main:   Ampersand: & Hex: & Copyright: ©
+ branch: Ampersand: & Hex: & Copyright: ©

  DOM parse - text
- main:   Ampersand: & Hex: & Copyright: ©
+ branch: Ampersand: & Hex: & Copyright: ©

  Serialize
- main:   <doc details="Ampersand: &amp; Hex: &amp; Copyright: ©"></doc>
+ branch: <doc details="Ampersand: &amp;#38; Hex: &amp;#x26; Copyright: &amp;#169;"></doc>

rexml

  SAX parse - attr
- main:   Ampersand: & Hex: & Copyright: ©
+ branch: Ampersand: & Hex: & Copyright: ©

  SAX parse - text     (unchanged)  Ampersand: & Hex: & Copyright: ©
  DOM parse - attr     (unchanged)  Ampersand: & Hex: & Copyright: ©
  DOM parse - text     (unchanged)  Ampersand: & Hex: & Copyright: ©
  Serialize            (unchanged)  <doc details="Ampersand: &amp;#38; Hex: &amp;#x26; Copyright: &amp;#169;"></doc>

libxml

  SAX parse - attr
- main:   Ampersand: & Hex: & Copyright: ©
+ branch: Ampersand: & Hex: & Copyright: ©

  SAX parse - text     (unchanged)  Ampersand: & Hex: & Copyright: ©
  DOM parse - attr     (unchanged)  Ampersand: & Hex: & Copyright: ©
  DOM parse - text     (unchanged)  Ampersand: & Hex: & Copyright: ©
  Serialize            (unchanged)  <doc details="Ampersand: &amp;#38; Hex: &amp;#x26; Copyright: &amp;#169;"></doc>