Fix: Support iteration and indexing for ModelSimple list responses

[Xyerophyte]

Problem

The DomainsResponse class (and other ModelSimple-based list response models) could not be iterated, causing ApiAttributeError when attempting common operations:

domains = domain_api.DomainApi(api_client).list_domains(id, version)
[domain for domain in domains]  # ❌ ApiAttributeError: DomainsResponse has no attribute '0'

Users had to access the internal _data_store directly, breaking the abstraction.

Root Cause

DomainsResponse inherits from ModelSimple and stores a list in its value attribute. When Python's iteration protocol calls __getitem__(0), the existing implementation only accepted string attribute names, not integer indices.

Solution

Enhanced the ModelSimple class in fastly/model_utils.py with sequence protocol support:

1. __getitem__ - Now accepts integers and slices, delegating to the underlying value list/tuple
2. __iter__ - Returns an iterator over the sequence in value
3. __len__ - Returns the length of the sequence in value

The implementation intelligently handles:

• Lists and tuples as sequences (iterable)
• Dicts and other objects as single items (to avoid iterating dict keys)
• Backward compatibility with existing attribute access

What Now Works

domains = domain_api.DomainApi(api_client).list_domains(id, version)

len(domains)              # ✓ Get count
domains[0]                # ✓ Index access  
domains[1:3]              # ✓ Slicing
list(domains)             # ✓ Convert to list
[d for d in domains]      # ✓ List comprehension
for domain in domains:    # ✓ Iteration
    print(domain)

Testing

• Added comprehensive test suite in tests/test_model_simple_sequence.py
• Tests cover iteration, indexing, slicing, len(), edge cases (empty, single item)
• All tests pass ✓

Documentation

• Updated CHANGELOG.md (v12.0.0 bug fixes section)
• Fix is backward-compatible with existing code

Impact

• Affects all ModelSimple instances that wrap sequences
• Makes list-based response models behave intuitively
• No breaking changes - purely additive functionality