Fix OP_RETURN Script Parsing and Code Quality Improvements

Description of Changes

This PR fixes a critical bug in Script._build_chunks() where scripts starting with 0x00 6a (OP_FALSE OP_RETURN) were incorrectly parsed, and includes several code quality improvements.

Bug Fix: OP_RETURN Handling

• Issue: Scripts containing OP_RETURN (0x6a) were not properly terminating script parsing. When OP_RETURN was encountered, the parser continued to parse subsequent bytes as opcodes instead of treating them as data.
• Fix: Implemented proper OP_RETURN handling following the TypeScript SDK pattern:
  • Added conditional block tracking (OP_IF, OP_NOTIF, OP_VERIF, OP_VERNOTIF / OP_ENDIF)
  • OP_RETURN outside conditional blocks now terminates parsing and treats all remaining bytes as data
  • OP_RETURN inside conditional blocks is treated as a normal opcode
• Impact: Scripts like 006a0454657374 now correctly parse as 2 chunks instead of 3

Code Quality Improvements

1. Reduced Cognitive Complexity:

   • Extracted _update_conditional_depth() helper method (reduced complexity in _build_chunks())
   • Extracted _handle_op_return() helper method
   • Extracted _read_push_data() helper method
   • Extracted _parse_opcode_token() and _parse_data_token() helpers in from_asm() method
   • All methods now meet SonarQube complexity requirements (≤15)

2. Field Renaming:

   • Renamed self.script to self._bytes to avoid confusion with the script parameter and improve code clarity

3. Test Improvements:

   • Added comprehensive test_op_return_chunk_parsing() test that verifies the bug fix
   • Reduced code duplication by extracting _create_spend_validator() helper function
   • Removed unused variable in test_r_puzzle()

Linked Issues / Tickets

Closes #[issue-number] - Script.chunks iterator mishandles / crashes on "safe" OP_RETURN scripts starting with 0x00 6a

Related to: #135

Testing Procedure

Unit Tests Added

• ✅ test_op_return_chunk_parsing(): Comprehensive test covering:
  • OP_FALSE OP_RETURN with data (the bug case: 006a0454657374)
  • OP_RETURN with data (no OP_FALSE prefix)
  • OP_RETURN with no data
  • OP_FALSE OP_RETURN with no data

Test Results

pytest tests/test_scripts.py -v

Result: 12/12 tests passed