fix(platform-demo-data): use the cart amount rule for demo shipping methods

[Dennis Garding (DennisGarding)]

1. Why is this change necessary?

Demo shipping methods could be assigned an unrelated rule because the provider selected the first rule returned by the database. Their availability rule should be deterministic.

2. What does this change do, exactly?

• Exposes the cart amount demo rule ID as a constant in RuleProvider.
• Assigns that rule explicitly to every demo shipping method.
• Adds a test confirming that an unrelated rule does not change the shipping methods’ availability rule.

3. Describe each step to reproduce the issue or behaviour.

1. Add another rule to the database.
2. Generate the platform demo data.
3. Check that every shipping method uses the cart amount demo rule as its availability rule.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.03%. Comparing base (673176e) to head (b9a7063).

Additional details and impacted files

@@             Coverage Diff              @@
##              trunk      #29      +/-   ##
============================================
+ Coverage     93.99%   94.03%   +0.03%     
+ Complexity      100       98       -2     
============================================
  Files            15       15              
  Lines          1582     1575       -7     
============================================
- Hits           1487     1481       -6     
+ Misses           95       94       -1     

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