Add unit tests for bytecode argument reading (args.rs)

[Cypher-CP0]

Summary

Closes #4240

Adds unit tests for the bytecode argument reading logic in core/engine/src/vm/opcode/args.rs, as requested in the linked issue.

Changes

Added the following test coverage inside the existing #[cfg(test)] mod tests block in args.rs:

Direct read function tests

• read_u8_direct — basic u8 read
• read_u16_little_endian — verifies correct little-endian byte ordering for u16
• read_u32_little_endian — verifies correct little-endian byte ordering for u32
• read_at_non_zero_offset — verifies reading from a non-zero offset advances pos correctly

Signed negative value tests

• read_i8_negative — reads -1 as i8
• read_i16_negative — reads -1 as i16 in little-endian
• read_i32_negative — reads i32::MIN from raw bytes

Operand edge cases

• register_operand_large_value — RegisterOperand with u32::MAX
• index_operand_mid_value — IndexOperand with a mid-range value
• address_max_value — Address with u32::MAX
• decode_at_non_zero_offset — decodes the second of two back-to-back encoded values

Primitive edge cases

• test_i8_max, test_i16_min_max, test_i32_max, test_u8_mid_value

Float special values

• test_f32_special_values — NaN, +inf, -inf for f32
• test_f64_special_values — NaN, +inf, -inf for f64

Additional tuple combinations

• test_triple_tuple_round_trip — 3-element tuples
• test_five_tuple_round_trip — 5-element tuples

Additional panic/truncation tests

• decode_u16_truncated_panics — 1 byte buffer for u16
• decode_u64_truncated_panics — 4 byte buffer for u64
• decode_at_offset_past_end_panics — offset + size exceeds buffer length

Additional ThinVec tests

• test_thin_vec_single_element — single element vec
• test_thin_vec_index_operand — ThinVec of IndexOperand with min/max values